LET
===

syntax: (let ({<variable> |
             (<variable> <value>) ...}) {<form> ...})

This function defines local variables, which can be
used in the <form>s. The start-value can be in every
form. If the start-value is not set, then NIL will
be used. LET returns the value of the last form.

example:       >global         ; defined global var.
               123
               >constant	; defined constant
               123
               >(let (x (y 789))
                     (list x global constant y))
               (NIL 123 123 789)