Steve Haflich from Franz kindly clarified to me my misunderstanding of special-operators and macros. In particular, he pointed out to me that implementations are free to implement special operators as macros. Steve writes: "I assure you that the set of special operators in ACL is _fixed+, and that there is way for you, the _user_, to extend that set. But this requirement of the ANS does not require that the set of cl-package symbols that return true under spcial-operator-p is not larger than the required operators in the table you quoted from http://www.franz.com/support/documentation/current/ansicl/subsubsu/specialf.... Now see here: http://www.franz.com/support/documentation/current/ansicl/subsubsu/macrofor.... specifically An implementation is free to implement a Common Lisp special operator as a macro. An implementation is free to implement any macro operator as a special operator, but only if an equivalent definition of the macro is also provided." He goes on to explain how this works in ACL: "In ACL user(31): (macro-function 'return) #<Function macroexpand-1-transform> so cl:return names _both_ a special operator and a macro. In our opinion, this is explicitly recognized by the ANS. (I think I remember being there when we of X3J13 voted this issue, but I also think I think I sometimes remember things that never occurred. Your mileage may vary.) Unfortunately, this seemingly innocent divergence from the standard breaks the code walker in ITERATE (and may break other code walkers, as well). I haven't examined this particular code walker, but I speculate that it is checking special-operator-p _before_ checking macro-function. A _portable_ walker should give the latter priority, which is what Franz believes the "implementation is free" codicil intends." Steve's understanding of the code walker in FREE-VARS is correct. Actually, over and above this, there seems to be a second bug in the code-walker: the check of (MACRO-FUNCTION (CAR FORM)) is not inside the branch for (SYMBOLP (CAR FORM)). So I believe that fixing this bug is not just a matter of pandering to ACL users, but actually a matter of fixing an incorrect code walker that could cause problems for arbitrary lisp implementations. I have attempted to fix this bug by moving the branch for MACRO-FUNCTION into the branch for SYMBOLP (CAR FORM) and moving it up before the check for SPECIAL-OPERATOR-P. See attached patch. I'd be obliged if you could review it for incorporation. It reduces the number of test failures in Allegro CL to the same as the number for SBCL: one. Best, R