question about comments in eval.c

[sen_ml at eccosys dot com]

in libguile/eval.c, i found the following comment:

   Memoization of variables and special forms is done while executing
   the code for the first time.

i think i understand what memoization of procedures and special forms
means, but i don't understand what memoization of variables is
supposed to mean.

can anyone shed some light on this?  (perhaps a pointer to an online
reference?)

(i only found a small amount of info in sicp about memoization, and
that didn't clear things up for me...)

thanks in advance.