If form is non-nil, this prints the memory allocation, in bytes, made during a function call. The symbol that form evaluates to is used to accumulate the amount of memory allocated between entering and exiting the traced function.
Note that this symbol continues to be used as an accumulator on subsequent calls to the traced function; the value is compounded, rather than over-written.
Consider the example below:
facfunction, set the value of
fac, and then evaluate
CL-USER 152 > $$fac-alloc
LispWorks User Guide and Reference Manual - 21 Dec 2011