define-modify-macro defines a macro named
name to read and write a place.
The arguments to the new macro are a place,
by the arguments that are supplied in lambda-list.
Macros defined with define-modify-macro
correctly pass the environment parameter to
When the macro is invoked, function
is applied to the old contents of the place
and the lambda-list arguments to obtain the new value,
and the place is updated to contain the result.
Except for the issue of avoiding multiple evaluation (see below), the expansion
of a define-modify-macro is equivalent to the following:
(defmacro name (reference . lambda-list)
(function ,reference ,arg1 ,arg2 ...)))
where arg1, arg2, ...,
are the parameters appearing in lambda-list;
appropriate provision is made for a rest parameter.
The subforms of the macro calls defined by define-modify-macro
are evaluated as specified in Section 18.104.22.168 Evaluation of Subforms to Places.
Documentation is attached as a documentation string
to name (as kind function)
and to the macro function.
If a define-modify-macro form appears as a top level form,
the compiler must store the macro definition at compile time,
so that occurrences of the macro later on in the file can be expanded correctly.