Provides a mechanism for globally affecting the macro expansion
of the indicated symbol.
Globally establishes an expansion function for the symbol macro
named by symbol.
The only guaranteed property of an expansion function for a symbol macro
is that when it is applied to the form and the environment it returns
the correct expansion. (In particular, it is implementation-dependent
whether the expansion is conceptually stored in the expansion function,
the environment, or both.)
Each global reference to symbol (i.e., not shadowed2 by a
binding for a variable or symbol macro named by
the same symbol) is expanded by the normal macro expansion process;
see Section 18.104.22.168.1 Symbols as Forms.
The expansion of a symbol macro is subject to further macro expansion
in the same lexical environment as the symbol macro reference,
exactly analogous to normal macros.
The consequences are unspecified if a special declaration is made for
symbol while in the scope of this definition (i.e., when it is not
shadowed2 by a binding for a variable
or symbol macro named by the same symbol).
Any use of setq to set the value of
while in the scope of this definition
is treated as if it were a setf.
psetq of symbol
is treated as if it were a psetf, and
is treated as if it were a setf of values.
A binding for a symbol macro can be shadowed2
by let or symbol-macrolet.