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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.