eql as test
Major Section: PROGRAMMING
(Substitute new old seq) is the result of replacing each occurrence
of old in seq, which is a list or a string, with new.
The guard for substitute requires that either seq is a string and
new is a character, or else: seq is a true-listp such that either
all of its members are eqlablep or old is eqlablep.
Substitute is a Common Lisp function. See any Common Lisp
documentation for more information. Since ACL2 functions cannot
take keyword arguments (though macros can), the test used in
substitute is eql.