If the call returns a number,
the match succeeds
and the returned number becomes the new current position.
(Assuming a subject <em>s</em> and current position <em>i</em>,
the returned number must be in the range <em>[i, len(s) + 1]</em>.)
If the call returns <b>true</b>,
the match succeeds without consuming any input.
(So, to return <b>true</b> is equivalent to return <em>i</em>.)
If the call returns <b>false</b>, <b>nil</b>, or no value,
the match fails.
</p>

<p>
Any extra values returned by the function 