I don't disagree. If debugging that sort of thing were intuitive, we'd all be coding in Prolog. It's like an invisible "if" hidden in every statement which branches elsewhere if there is no match. @(assert) can help; if you know that something that follows must match, you get an exception if it doesn't. I used asserts in the man page checker:

http://www.kylheku.com/cgit/txr/tree/checkman.txr