(VAR x y z w)
(RULES
f(f(s(x),0),f(y,z)) -> f(f(y,z),f(y,s(z))) 
f(f(s(x),s(y)),f(z,w)) -> f(f(x,y),f(z,w)) 
)
(COMMENT coding of while x<y do y := y+1 )