If c is a conversion, then CONV_RULE c is an inference rule that applies
c to the conclusion of a theorem.  That is, if c maps a term "t" to the
theorem |- t = t', then the rule CONV_RULE c infers |- t' from the
theorem |- t.  More precisely, if c "t" returns A' |- t = t', then:
       A |- t
   --------------  CONV_RULE c
    A u A' |- t'