ZERO
fold-left($f as function(item()*, item()) as item()*, $zero as item()*, $seq as item()*) → item()*
fold-right($f as function(item(), item()*) as item()*, $zero as item()*, $seq as item()*) → item()*
sum($arg as xs:anyAtomicType*, $zero as xs:anyAtomicType?) → xs:anyAtomicType?
ZERO-OR-ONE
zero-or-one($arg as item()*) → item()?