expansion - a form.

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 3.1.2.1.1 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 the symbol 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 multiple-value-setq is treated as if it were a setf of values.

A binding for a symbol macro can be shadowed₂ by let or symbol-macrolet.

(defvar *things* (list 'alpha 'beta 'gamma))  *THINGS*

(define-symbol-macro thing1 (first *things*))   THING1
(define-symbol-macro thing2 (second *things*))   THING2
(define-symbol-macro thing3 (third *things*))   THING3

thing1   ALPHA
(setq thing1 'ONE)   ONE
*things*  (ONE BETA GAMMA)
(multiple-value-setq (thing2 thing3) (values 'two 'three))   TWO
thing3   THREE
*things*  (ONE TWO THREE)

(list thing2 (let ((thing2 2)) thing2))  (TWO 2)