Applying the tactic
   ASSUME_TAC (SIMP_PROVE arith_ss [] ``x < y ==> x < y + 6``)
 
to the goal ?- x + y = 10 yields the new goal
   x < y ==> x < y + 6 ?- x + y = 10
 
Using SIMP_PROVE here allows ASSUME_TAC to add a new fact, where
the equality with truth that SIMP_CONV would produce would be less
useful.