An enumeration type defines a set of permitted
xs:string values. For example, an enumeration
type might be defined as:
enum("red", "green", "blue")
The values conforming to an enumeration type are strings: the expression
$S instance of enum(A, B, C)
is true if
$S is an instance of
xs:string (note: not
xs:anyURI and is equal (under codepoint collation) to one of A, B, or C.
A typical use case is in a function signature. For example the function signature of
could be restated as
function($s as xs:string, $form as enum("NFC", "NFD", "NFKC", "NFKD")) as xs:string.