Returns the first item in a sequence.
fn:head( $arg as item()*item()?The function returns the value of the expression $arg[1]
The expression fn:head(1 to 5) returns 1.
The expression fn:head(("a", "b", "c")) returns "a".
The expression fn:head(()) returns ().
The expression fn:head([1,2,3]) returns [1,2,3].
If $arg is the empty sequence, the empty sequence is returned. Otherwise
the first item in the sequence is returned.
