main(x) = (s,t) where (s,t) = f(x)

f(C1(x,y)) = (C(a,b),C(s,t))
	   where (a,b) = f(x)
	         (s,t) = f(y)
f(C2(x,y)) = (C(a,s),C(b,t))
	   where (a,b) = f(x)
	         (s,t) = f(y)
f(C0)      = (E,F)
