Returns the arity of the function identified by a function item.
fn:function-arity( $func as function(*)xs:integerThe fn:function-arity function returns the arity (number of arguments) of
the function identified by $func.
The expression fn:function-arity(fn:substring#2) returns 2.
The expression fn:function-arity(function($node){name($node)}) returns 1.
The expression let $initial := fn:substring(?, 1, 1) return
fn:function-arity($initial) returns 1.