valA (a :-: b) s st = ((x - y),st2) where (x,st1) = evalA a s st
                                           (y,st2) = evalA b s st1 

evalA (a :-: b) s st = ((x - y),st2) where (x,st1) = evalA a s st
                                           (y,st2) = evalA b s st1 
evalA (a :-: b) s st = ((x - y),st2) where (x,st1) = evalA a s st
                                           (y,st2) = evalA b s st1 