Class ExpressionTool


  • public class ExpressionTool
    extends java.lang.Object
    This class, ExpressionTool, contains a number of useful static methods for manipulating expressions. Most importantly, it provides the factory method make() for constructing a new expression
    • Method Detail

      • make

        public static Expression make​(java.lang.String expression,
                                      StaticContext env,
                                      int start,
                                      int terminator,
                                      CodeInjector codeInjector)
                               throws XPathException
        Parse an XPath expression. This performs the basic analysis of the expression against the grammar, it binds variable references and function calls to variable definitions and function definitions, and it performs context-independent expression rewriting for optimization purposes.
        Parameters:
        expression - The expression (as a character string)
        env - An object giving information about the compile-time context of the expression
        start - position of the first significant character in the expression
        terminator - The token that marks the end of this expression; typically Token.EOF, but may for example be a right curly brace
        codeInjector - true allows injection of tracing, debugging, or performance monitoring code; null if not required
        Returns:
        an object of type Expression
        Throws:
        XPathException - if the expression contains a static error
      • setDeepRetainedStaticContext

        public static void setDeepRetainedStaticContext​(Expression exp,
                                                        RetainedStaticContext rsc)
        Ensure that every node in the expression tree has a retained static context. A node that already has a retained static context will propagate it to its children.
        Parameters:
        exp - the root of the expression tree
        rsc - the retained static context to be applied to nodes that do not already have one, and that do not have an ancestor with an existing retained static context.
      • copyLocationInfo

        public static void copyLocationInfo​(Expression from,
                                            Expression to)
        Copy location information (the line number and reference to the container) from one expression to another
        Parameters:
        from - the expression containing the location information
        to - the expression to which the information is to be copied
      • unsortedIfHomogeneous

        public static Expression unsortedIfHomogeneous​(Expression exp,
                                                       boolean forStreaming)
                                                throws XPathException
        Remove unwanted sorting from an expression, at compile time, if and only if it is known that the result of the expression will be homogeneous (all nodes, or all atomic values). This is done when we need the effective boolean value of a sequence: the EBV of a homogenous sequence does not depend on its order, but this is not true when atomic values and nodes are mixed: (N, AV) is true, but (AV, N) is an error.
        Parameters:
        exp - the expression to be optimized
        forStreaming - true if streamed evaluation of the expression is required
        Returns:
        the expression after rewriting
        Throws:
        XPathException - if a static error is found while doing the rewrite
      • injectCode

        public static Expression injectCode​(Expression exp,
                                            CodeInjector injector)
        Inject extra code into an expression, recursively, by applying a supplied CodeInjector to every expression (and FLWOR clause) in the subtree
        Parameters:
        exp - the expression to be augmented with injected code
        injector - the code injector
        Returns:
        the augmented expression
      • lazyEvaluator

        public static Evaluator lazyEvaluator​(Expression exp,
                                              boolean repeatable)
        Get an expression evaluator to be used when lazy evaluation of an expression is preferred. This method is called at compile time, after all optimizations have been done, to determine the preferred strategy for lazy evaluation, depending on the type of expression.
        Parameters:
        exp - the expression to be evaluated
        repeatable - true if the returned value needs to be readable more than once
        Returns:
        an expression evaluator
      • eagerEvaluator

        public static Evaluator eagerEvaluator​(Expression exp)
        Get the evaluator to be used when eager evaluation of an expression is preferred. This method is called at compile time, after all optimizations have been done, to determine the preferred strategy for lazy evaluation, depending on the type of expression.
        Parameters:
        exp - the expression to be evaluated
        Returns:
        an integer constant identifying the evaluation mode
      • lazyEvaluate

        public static Sequence lazyEvaluate​(Expression exp,
                                            XPathContext context,
                                            boolean repeatable)
                                     throws XPathException
        Do lazy evaluation of an expression. This will return a value, which may optionally be a SequenceIntent, which is a wrapper around an iterator over the value of the expression.
        Parameters:
        exp - the expression to be evaluated
        context - the run-time evaluation context for the expression. If the expression is not evaluated immediately, then parts of the context on which the expression depends need to be saved as part of the Closure
        repeatable - an indication of how the value will be used. The value false indicates that the value is only expected to be used once, so that there is no need to keep it in memory. The value true indicates multiple references, so the value will be saved when first evaluated.
        Returns:
        a value: either the actual value obtained by evaluating the expression, or a Closure containing all the information needed to evaluate it later
        Throws:
        XPathException - if any error occurs in evaluating the expression
      • eagerEvaluate

        public static GroundedValue eagerEvaluate​(Expression exp,
                                                  XPathContext context)
                                           throws XPathException
        Evaluate an expression now; lazy evaluation is not permitted in this case
        Parameters:
        exp - the expression to be evaluated
        context - the run-time evaluation context
        Returns:
        the result of evaluating the expression
        Throws:
        XPathException - if any dynamic error occurs evaluating the expression
      • markTailFunctionCalls

        public static int markTailFunctionCalls​(Expression exp,
                                                StructuredQName qName,
                                                int arity)
        Scan an expression to find and mark any recursive tail function calls
        Parameters:
        exp - the expression to be analyzed
        qName - the name of the containing function
        arity - the arity of the containing function
        Returns:
        0 if no tail call was found; 1 if a tail call to a different function was found; 2 if a tail call to the specified function was found. In this case the UserFunctionCall object representing the tail function call will also have been marked as a tail call.
      • indent

        public static java.lang.String indent​(int level)
        Construct indent string, for diagnostic output
        Parameters:
        level - the indentation level (the number of spaces to return)
        Returns:
        a string of "level*2" spaces
      • contains

        public static boolean contains​(Expression a,
                                       Expression b)
        Ask whether one expression is a subexpression of another
        Parameters:
        a - the containing expression
        b - the putative contained expression
        Returns:
        true if and only if b is a subexpression (at some level) of a
      • containsLocalParam

        public static boolean containsLocalParam​(Expression exp)
        Determine whether an expression contains a LocalParamSetter subexpression
        Parameters:
        exp - the expression to be tested
        Returns:
        true if the exprssion contains a local parameter setter
      • containsLocalVariableReference

        public static boolean containsLocalVariableReference​(Expression exp)
        Determine whether an expression contains a local variable reference, other than a reference to a local variable whose binding occurs within this expression
        Parameters:
        exp - the expression to be tested
        Returns:
        true if the expression contains a local variable reference to a local variable declared outside this expression.
      • contains

        public static boolean contains​(Expression exp,
                                       boolean sameFocusOnly,
                                       java.util.function.Predicate<Expression> predicate)
        Test whether a given expression is, or contains, at any depth, an expression that satisfies a supplied condition
        Parameters:
        exp - the given expression
        sameFocusOnly - if true, only subexpressions evaluated in the same focus are searched. If false, all subexpressions are searched
        predicate - the condition to be satisfied
        Returns:
        true if the given expression is or contains an expression that satisfies the condition.
      • changesXsltContext

        public static boolean changesXsltContext​(Expression exp)
        Determine whether an expression possible calls (directly or indirectly) xsl:result-document, or has other context dependencies that prevent function inlining, which we assume is true if it contains a template call. A call on result-document disqualifies the function from being inlined, because error XTDE1480 would then not be detected. We don't have to worry about functions containing further function calls because they are disqualified from inlining anyway.
        Parameters:
        exp - the expression to be tested
        Returns:
        true if there is any possibility that calling the function might result in a call on xsl:result-document
      • isLoopingSubexpression

        public static boolean isLoopingSubexpression​(Expression child,
                                                     Expression ancestor)
        Return true if a given expression is evaluated repeatedly when a given ancestor expression is evaluated once
        Parameters:
        child - the expression to be tested
        ancestor - the ancestor expression. May be null, in which case the search goes all the way to the base of the stack.
        Returns:
        true if the current expression is evaluated repeatedly
      • hasLoopingSubexpression

        public static boolean hasLoopingSubexpression​(Expression parent,
                                                      Expression child)
      • getFocusSettingContainer

        public static Expression getFocusSettingContainer​(Expression exp)
        Get the focus-setting container of an expression *

        Note, this always returns an expression or null. Unlike the like-named concept in the spec, it can't return a component such as a template or an attribute set.

        Parameters:
        exp - the expression whose focus-setting container is required
        Returns:
        the focus-setting container, or null if the focus for the expression is the same as the focus for the containing component as a whole
      • getContextDocumentSettingContainer

        public static Expression getContextDocumentSettingContainer​(Expression exp)
        Get the context-document-setting container of an expression. This is the same as the focus-setting container, except when the context-setting operand is a single-document node-set, in which case we can go up one more level. For example, given /a/b[/d/e = 3], the predicate /d/e = 3 can be loop-lifted from the filter expression, because it is known that the base expression /a/b is a single-document node-set.
        Parameters:
        exp - the expression whose document-context-setting container is required
        Returns:
        the context-document-setting container, or null if the focus for the expression is the same as the focus for the containing component as a whole
      • resetStaticProperties

        public static void resetStaticProperties​(Expression exp)
        Reset the static properties for the current expression and for all its containing expressions. This should be done whenever the expression is changed in a way that might affect the properties. It causes the properties to be recomputed next time they are needed.
        Parameters:
        exp - the expression whose properties are to be reset; the method also resets local properties for all its ancestor expressions.
      • getAxisNavigation

        public static int getAxisNavigation​(Expression exp)
        Get the axis followed by this expression, if there is one. For an axis expression or a filtered axis expression this returns the axis. For a union/intersect/except expression it returns the common axis if they all use the same axis. Where no axis can be discerned, return -1
        Returns:
        the axis followed by this expression if applicable (as an AxisInfo constant) or -1 otherwise.
      • equalOrNull

        public static boolean equalOrNull​(java.lang.Object x,
                                          java.lang.Object y)
        Return true if two objects are equal or if both are null
        Parameters:
        x - the first object
        y - the second object
        Returns:
        true if both x and y are null or if x.equals(y)
      • getIteratorFromProcessMethod

        public static SequenceIterator getIteratorFromProcessMethod​(Expression exp,
                                                                    XPathContext context)
                                                             throws XPathException
        Helper method to construct an iterator over the results of the expression when all that the expression itself offers is a process() method. This builds the entire results of the expression as a sequence in memory and then iterates over it.
        Parameters:
        context - the dynamic evaluation context
        Returns:
        an iterator over the results of the expression
        Throws:
        XPathException - if a dynamic error occurs
      • getItemFromProcessMethod

        public static Item getItemFromProcessMethod​(Expression exp,
                                                    XPathContext context)
                                             throws XPathException
        Helper method to construct an item representing the results of the expression when all that the expression itself offers is a process() method.
        Parameters:
        context - the dynamic evaluation context
        Returns:
        an iterator over the results of the expression
        Throws:
        XPathException - if a dynamic error occurs
      • allocateSlots

        public static int allocateSlots​(Expression exp,
                                        int nextFree,
                                        SlotManager frame)
        Allocate slot numbers to range variables
        Parameters:
        exp - the expression whose range variables need to have slot numbers assigned
        nextFree - the next slot number that is available for allocation
        frame - a SlotManager object that is used to track the mapping of slot numbers to variable names for debugging purposes. May be null.
        Returns:
        the next unallocated slot number.
      • effectiveBooleanValue

        public static boolean effectiveBooleanValue​(SequenceIterator iterator)
                                             throws XPathException
        Determine the effective boolean value of a sequence, given an iterator over the sequence
        Parameters:
        iterator - An iterator over the sequence whose effective boolean value is required
        Returns:
        the effective boolean value
        Throws:
        XPathException - if a dynamic error occurs
      • effectiveBooleanValue

        public static boolean effectiveBooleanValue​(Item item)
                                             throws XPathException
        Determine the effective boolean value of a single item
        Parameters:
        item - the item whose effective boolean value is required
        Returns:
        the effective boolean value
        Throws:
        XPathException - if a dynamic error occurs
      • ebvError

        public static void ebvError​(java.lang.String reason)
                             throws XPathException
        Report an error in computing the effective boolean value of an expression
        Parameters:
        reason - the nature of the error
        Throws:
        XPathException - always
      • ebvError

        public static void ebvError​(java.lang.String reason,
                                    Expression cause)
                             throws XPathException
        Report an error in computing the effective boolean value of an expression
        Parameters:
        reason - the nature of the error
        Throws:
        XPathException - always
      • dependsOnFocus

        public static boolean dependsOnFocus​(Expression exp)
        Ask whether an expression has a dependency on the focus
        Parameters:
        exp - the expression
        Returns:
        true if the value of the expression depends on the context item, position, or size
      • dependsOnVariable

        public static boolean dependsOnVariable​(Expression exp,
                                                Binding[] bindingList)
        Determine whether an expression depends on any one of a set of variables
        Parameters:
        exp - the expression being tested
        bindingList - the set of variables being tested
        Returns:
        true if the expression depends on one of the given variables
      • gatherReferencedVariables

        public static void gatherReferencedVariables​(Expression e,
                                                     java.util.List<Binding> list)
        Gather a list of all the variable bindings on which a given expression depends. In the light of spec bug 29590, this only considers variable references that are unconditionally evaluated: that is, it ignores variable references appearing in the then/else branches of a conditional or equivalent. Cycles involving conditional evaluation are detected dynamically.
        Parameters:
        e - the expression being tested
        list - a list to which the bindings are to be added. The items in this list must implement Binding
      • refersToVariableOrFunction

        public static boolean refersToVariableOrFunction​(Expression exp)
        Determine whether the expression contains any variable references or calls to user-written functions
        Parameters:
        exp - the expression being tested
        Returns:
        true if the expression includes a variable reference or function call, or an XSLT construct equivalent to a function call (e.g call-template). Also returns true if the expression includes a variable binding element, as (a) this is likely to mean it also contains a reference, and (b) it also needs to be caught on the same paths.
      • isCallOnSystemFunction

        public static boolean isCallOnSystemFunction​(Expression e,
                                                     java.lang.String localName)
      • callsFunction

        public static boolean callsFunction​(Expression exp,
                                            StructuredQName qName,
                                            boolean sameFocusOnly)
        Determine whether an expression contains a call on the function with a given name
        Parameters:
        exp - The expression being tested
        qName - The name of the function
        sameFocusOnly - If true, only expressions with the same (top-level) focus are searched
        Returns:
        true if the expression contains a call on the function
      • containsSubexpression

        public static boolean containsSubexpression​(Expression exp,
                                                    java.lang.Class<? extends Expression> subClass)
        Determine whether an expression has a subexpression of a given implementation class
        Parameters:
        exp - The expression being tested
        subClass - The implementation class of the sought subexpression
        Returns:
        true if the expression contains a subexpression that is an instance of the specified class
      • gatherCalledFunctions

        public static void gatherCalledFunctions​(Expression e,
                                                 java.util.List<UserFunction> list)
        Gather a list of all the user-defined functions which a given expression calls directly
        Parameters:
        e - the expression being tested
        list - a list of the functions that are called. The items in this list must be objects of class UserFunction
      • gatherCalledFunctionNames

        public static void gatherCalledFunctionNames​(Expression e,
                                                     java.util.List<SymbolicName> list)
        Gather a list of the names of the user-defined functions which a given expression calls directly
        Parameters:
        e - the expression being tested
        list - a list of the functions that are called. The items in this list are strings in the format "{uri}local/arity"
      • optimizeComponentBody

        public static Expression optimizeComponentBody​(Expression body,
                                                       Compilation compilation,
                                                       ExpressionVisitor visitor,
                                                       ContextItemStaticInfo cisi,
                                                       boolean extractGlobals)
                                                throws XPathException
        Optimize the implementation of a component such as a function, template, or global variable
        Parameters:
        body - the expression forming the body of the component
        compilation - the current compilation. May be null.
        visitor - the expression visitor
        cisi - information about the context item for evaluation of the component body
        extractGlobals - true if constant expressions are to be extracted as global variables
        Returns:
        the optimized expression body
        Throws:
        XPathException - if anything goes wrong
      • computeEvaluationModesForUserFunctionCalls

        public static void computeEvaluationModesForUserFunctionCalls​(Expression exp)
                                                               throws XPathException
        Compute argument evaluation modes for all calls on user defined functions with a specified expression
        Throws:
        XPathException
      • clearStreamabilityData

        public static void clearStreamabilityData​(Expression exp)
                                           throws XPathException
        Clear all computed streamability properties for an expression and its contained subtree
        Parameters:
        exp - the expression whose streamability data is to be reset
        Throws:
        XPathException - should not happen
      • resetPropertiesWithinSubtree

        public static void resetPropertiesWithinSubtree​(Expression exp)
        Reset cached static properties within a subtree, meaning that they have to be recalulated next time they are required
        Parameters:
        exp - the root of the subtree within which static properties should be reset
      • resolveCallsToCurrentFunction

        public static Expression resolveCallsToCurrentFunction​(Expression exp)
        Resolve calls to the XSLT current() function within an expression
        Parameters:
        exp - the expression within which calls to current() should be resolved
        Returns:
        the expression after resolving calls to current()
      • gatherVariableReferences

        public static void gatherVariableReferences​(Expression exp,
                                                    Binding binding,
                                                    java.util.List<VariableReference> list)
        Get a list of all references to a particular variable within a subtree
        Parameters:
        exp - the expression at the root of the subtree
        binding - the variable binding whose references are sought
        list - a list to be populated with the references to this variable
      • processExpressionTree

        public static boolean processExpressionTree​(Expression root,
                                                    java.lang.Object result,
                                                    ExpressionAction action)
                                             throws XPathException
        Process every node on a subtree of the expression tree using a supplied action.
        Parameters:
        root - the root of the subtree to be processed
        result - an arbitrary object that is passed to each action call and that can be updated to gather results of the processing
        action - an action to be performed on each node of the expression tree. Processing stops if any action returns the value true
        Returns:
        true if any call on the action operand returned true
        Throws:
        XPathException - if the callback throws an error
      • replaceSelectedSubexpressions

        public static boolean replaceSelectedSubexpressions​(Expression exp,
                                                            ExpressionTool.ExpressionSelector selector,
                                                            Expression replacement,
                                                            boolean mustCopy)
        Replace all selected subexpressions within a subtree
        Parameters:
        exp - the expression at the root of the subtree
        selector - callback to determine whether a subexpression is selected
        replacement - the expression to be used in place of the variable reference
        mustCopy - if true, the replacement expression must be copied before use
        Returns:
        true if a replacement has been performed (in which case the replacement expression must be copied before being used again).
      • replaceVariableReferences

        public static void replaceVariableReferences​(Expression exp,
                                                     Binding binding,
                                                     Expression replacement,
                                                     boolean mustCopy)
        Replace all references to a particular variable within a subtree
        Parameters:
        exp - the expression at the root of the subtree
        binding - the variable binding whose references are sought
        replacement - the expression to be used in place of the variable reference
        mustCopy - true if the replacement expression must be copied before use
      • getReferenceCount

        public static int getReferenceCount​(Expression exp,
                                            Binding binding,
                                            boolean inLoop)
        Determine how often a variable is referenced. This is the number of times it is referenced at run-time: so a reference in a loop counts as "many". This code currently handles local variables (Let expressions) and function parameters. It is not currently used for XSLT template parameters. It's not the end of the world if the answer is wrong (unless it's wrongly given as zero), but if wrongly returned as 1 then the variable will be repeatedly evaluated.
        Parameters:
        exp - the expression within which variable references are to be counted
        binding - identifies the variable of interest
        inLoop - true if the expression is within a loop, in which case a reference counts as many. This should be set to false on the initial call, it may be set to true on an internal recursive call
        Returns:
        the number of references. The interesting values are 0, 1, "many" (represented by any value >1), and the special value FILTERED, which indicates that there are multiple references and one or more of them is of the form $x[....] indicating that an index might be useful.
      • expressionSize

        public static int expressionSize​(Expression exp)
        Get the size of an expression tree (the number of subexpressions it contains)
        Parameters:
        exp - the expression whose size is required
        Returns:
        the size of the expression tree, as the number of nodes
      • rebindVariableReferences

        public static void rebindVariableReferences​(Expression exp,
                                                    Binding oldBinding,
                                                    Binding newBinding)
        Rebind all variable references to a binding
        Parameters:
        exp - the expression whose contained variable references are to be rebound
        oldBinding - the old binding for the variable references
        newBinding - the new binding to which the variables should be rebound
      • makePathExpression

        public static Expression makePathExpression​(Expression start,
                                                    Expression step)
        Make a mapping expression. The resulting expression will include logic to check that the first operand returns nodes, and that the expression as a whole is homogeneous, unless the caller requests otherwise.
        Parameters:
        start - the start expression (the first operand of "/")
        step - the step expression (the second operand of "/")
        Returns:
        the resulting expression.
      • findOperand

        public static Operand findOperand​(Expression parentExpression,
                                          Expression childExpression)
        Find the operand corresponding to a particular child expression
        Parameters:
        parentExpression - the parent expression
        childExpression - the child expression
        Returns:
        the relevant operand, or null if not found
      • unfilteredExpression

        public static Expression unfilteredExpression​(Expression exp,
                                                      boolean allowPositional)
        Get the expression that remains after removing any filter predicates
        Parameters:
        exp - the expression to be examined
        allowPositional - true if positional predicates are allowed
        Returns:
        the expression underlying exp after removing any predicates
      • tryToFactorOutDot

        public static Expression tryToFactorOutDot​(Expression exp,
                                                   ItemType contextItemType)
        Try to factor out dependencies on the context item, by rewriting an expression f(.) as let $dot := . return f($dot). This is not always possible, for example where f() is an extension function call that uses XPathContext as an implicit argument. However, doing this increases the chances of distributing a "where" condition in a FLWOR expression to the individual input sequences selected by the "for" clauses.
        Parameters:
        exp - the expression from which references to "." should be factored out if possible
        contextItemType - the static type of the context item
        Returns:
        either the expression, after binding "." to a local variable and replacing all references to it; or null, if no changes were made.
      • factorOutDot

        public static boolean factorOutDot​(Expression exp,
                                           Binding variable)
        Replace references to the context item with references to a variable
        Parameters:
        exp - the expression in which the replacement is to take place
        variable - the declaration of the variable
        Returns:
        true if replacement has taken place (at any level)
      • inlineVariableReferences

        public static boolean inlineVariableReferences​(Expression expr,
                                                       Binding binding,
                                                       Expression replacement)
        Inline variable references.
        Parameters:
        expr - the expression containing the variable references to be inlined
        binding - the variable binding to be inlined
        replacement - the expression to be used as a replacement for the variable reference
        Returns:
        true if any replacement was carried out within the subtree of this expression
      • inlineVariableReferencesInternal

        public static boolean inlineVariableReferencesInternal​(Expression expr,
                                                               Binding binding,
                                                               Expression replacement)
      • replaceTrivialCallsToCurrent

        public static boolean replaceTrivialCallsToCurrent​(Expression expr)
        Replace trivial calls to current() by a context item expression (.).
        Parameters:
        expr - the expression potentially containing the calls to be replaced
        Returns:
        true if any replacement was carried out within the subtree of this expression
      • replaceCallsToCurrent

        public static boolean replaceCallsToCurrent​(Expression expr,
                                                    LocalBinding binding)
        Replace calls to current() by a variable reference.
        Parameters:
        expr - the expression potentially containing the calls to be replaced
        binding - the variable binding to be referenced
        Returns:
        true if any replacement was carried out within the subtree of this expression
      • isNotAllowedInUpdatingContext

        public static boolean isNotAllowedInUpdatingContext​(Expression exp)
        Determine whether the expression is either an updating expression, or an expression that is permitted in a context where updating expressions are allowed
        Parameters:
        exp - the expression under test
        Returns:
        true if the expression is neither an updating expression, nor an empty sequence, nor a call on error()
      • getCurrentDirectory

        public static java.lang.String getCurrentDirectory()
      • getBaseURI

        public static java.net.URI getBaseURI​(StaticContext env,
                                              javax.xml.transform.SourceLocator locator,
                                              boolean fail)
                                       throws XPathException
        Determine the base URI of an expression, so that it can be saved on the expression tree for use when the expression is evaluated
        Parameters:
        env - the static context
        locator - location of the expression for error messages
        fail - if true, the method throws an exception when there is no absolute base URI; otherwise, the method returns null
        Returns:
        the absolute base URI of the expression
        Throws:
        XPathException - if an error occurs
      • parenthesize

        public static java.lang.String parenthesize​(Expression exp)
        Display an expression adding parentheses if it is possible they are necessary because the expression has sub-expressions
        Parameters:
        exp - the expression to be displayed
        Returns:
        a representation of the expression in parentheses
      • parenthesizeShort

        public static java.lang.String parenthesizeShort​(Expression exp)
      • validateTree

        public static void validateTree​(Expression exp)
      • isLocalConstructor

        public static boolean isLocalConstructor​(Expression child)
        Ask whether a supplied expression is a nested node constructor. That is, return true if the child expression creates nodes that will only be used as children of some parent node (meaning that they never need to be copied).
        Parameters:
        child - child expression to be tested
        Returns:
        true if the node constructed by the child expression does not need to be copied.