If c is a conversion that maps a term "t2" to the theorem |- t2 = t2',
then the rule RIGHT_CONV_RULE c infers |- t1 = t2' from the theorem
|- t1 = t2.  That is, if  c "t2" returns A' |- t2 = t2', then:
       A |- t1 = t2
   ---------------------  RIGHT_CONV_RULE c
    A u A' |- t1 = t2'