Unused variables compiler warning

[Moon at MIT-AI (David A. Moon)]

In system 16, warnings for variables bound but not referenced are given
for arguments as well as local variables.  This can be a pain when you
have automatically-generated functions.  There are two ways to get rid
of this warning that you may not know about.  One is to name the offending
variable IGNORE.  The other is to put the variable at the front of the
function body; this calls for the variable's value but doesn't do anything
with it.  This gets rid of the warning without affecting the generated code,
and provides a convenient place to put a comment as to why the argument
is ignored.