(VAR x y )
(RULES 
        f(s(x),y) -> f(-(s(x),y),y)
        +(0,y) ->= y
        +(s(x),y) ->= s(+(x,y))
        -(x,0) ->= x
        -(0,y) ->= 0
        -(s(x),s(y)) ->= -(x,y)
        f(x,y) ->= f(x,+(x,y))
)