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There are some subtleties to be aware of when writing iterate clauses.
First, the code returned by your macros may be nconc'ed into a
list, so you should always returned freshly consed lists, rather than
constants. Second, iterate matches clauses by using eq on the
first symbol and string= on the subsequent ones, so the package
of the first symbol of a clause is relevant. All of the clauses in
this manual have their first word in the iterate package. You can use
the package system in the usual way to shadow iterate clauses without
replacing them.