Generate code that evaluates certain expressions only once. This is used in macros, for computing expansions. VARIABLE-LIST is a list of symbols, whose values are subexpressions to be substituted into a larger expression. BODY is what uses those symbols' values and constructs the larger expression. ONCE-ONLY modifies BODY so that it constructs a different expression, which when run will evaluate the subsexpressions only once, save the values in temporary variables, and use those from then on. Example: (DEFMACRO DOUBLE (ARG) `(+ ,ARG ,ARG)) expands into code that computes ARG twice. (DEFMACRO DOUBLE (ARG) (ONCE-ONLY (ARG) `(+ ,ARG ,ARG))) will not.