TRIVIAL-CONSEQUENCE a rule at all. We could do this with
:rule-classes 
nil.
This Warning alerts us to the fact that when treated as a rewrite
rule, the new rule TRIVIAL-CONSEQUENCE, rewrites terms of the same form
as a rule we have already proved, namely ASSOCIATIVITY-OF-APP.
When you see this warning you should think about your rules!
In the current case, it would be a good idea not to make
TRIVIAL-CONSEQUENCE a rule at all. We could do this with
:rule-classes nil.
ACL2 proceeds to try to prove the theorem, even though it printed
some warnings. The basic assumption in ACL2 is that the user
understands what he or she is doing but may need a little reminding
just to manage a complicated set of facts.
