f(x,0) = x
f(x,i(x)) = 0
f(f(x,y),z) = f(x,f(y,z))
f(f(x,x),x) = 0
h(x,y) = f(f(x,y),f(i(x),i(y)))
!preorder i > f > h > 0
h(h(c1,c2),c2) != 0