symbol-macrolet provides a mechanism for
affecting the macro expansion environment for symbols.
symbol-macrolet lexically establishes expansion functions
for each of the symbol macros named by symbols.
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 reference to symbol as a variable within the lexical scope
of symbol-macrolet 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 invocation, exactly
analogous to normal macros.
Exactly the same declarations are allowed as for let
with one exception: symbol-macrolet signals an error
if a special declaration names one of the symbols
being defined by symbol-macrolet.
When the forms of the symbol-macrolet form are expanded,
any use of setq to set the value of one of the specified variables
is treated as if it were a setf.
psetq of a symbol defined as a symbol macro
is treated as if it were a psetf, and
is treated as if it were a setf of values.
The use of symbol-macrolet can be shadowed by let.
In other words, symbol-macrolet only substitutes for occurrences
of symbol that would be in the scope of a lexical binding of
symbol surrounding the forms.