PFORALL_EQ : (term -> thm -> thm)
    A |- t1 = t2
whose conclusion is an equation between boolean terms:
    PFORALL_EQ
returns the theorem:
    A |- (!p. t1) = (!p. t2)
unless any of the variables in p is free in any of the assumptions.
          A |- t1 = t2
   --------------------------  PFORALL_EQ "p"      [where p is not free in A]
    A |- (!p. t1) = (!p. t2)