A problem common to many powerful programming languages arises when one has to determine what values to assign to free variables in functions. Different implementational approaches which attempt to solve the problem are considered. The discussion concentrates on LISP implementations and points out why most current LISP systems are not as general as the original LISP 1.5 system. Readers not familiar with LISP should be able to read this paper without difficulty since we have tried to couch the argument in ALGOL-like terms as much as possible.
