[erlang-questions] guard clause oddity, maybe bug (in my brain)?

[jim rosenblum]

I have a function:

walk ([I|Is], [Elt|_]=Ts) when (not is_tuple(Elt)) andalso ?IS_INDEX(I) ->
   Element = index_to_element(I,Ts),
   case
       ... snip ...
   end.

This function clause gets executed even though Elt is a tuple.

I have a break point within this function clause. I am looking at the
debugger at the break-point within the clause, and I can plainly see that
Elt
IS a tuple: within the debugger I can evaluate is_tuple(Elt) and it returns
true,
I can evaluate (not is_tuple(Elt)) and correctly get FALSE.

So.... how can I be in this clause when the guard evaluates to false?

Thoughts?
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