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fun length [] = 0
   | length (head::tail) = 1 + length tail;

sub length {
...
    return 1 + length(@tail);
}

member (X, [])    -> false;
member (X, [X|_]) -> true;
member (X, [_|T]) -> member(X, T).

sub member {
...
    return member($X, @tail) if $X != $head;
}