EQ_MPThm.EQ_MP : thm -> thm -> thm
Equality version of the Modus Ponens rule.
When applied to theorems A1 |- t1 = t2 and
A2 |- t1, the inference rule EQ_MP returns the
theorem A1 u A2 |- t2.
A1 |- t1 = t2 A2 |- t1
-------------------------- EQ_MP
A1 u A2 |- t2
Fails unless the first theorem is equational and its left side is the
same as the conclusion of the second theorem (and is therefore of type
bool), up to alpha-conversion.