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'
Note that if the conversion c returns a theorem with assumptions,
then the resulting inference rule adds these to the assumptions of the
theorem it returns.