Cases for variables and pairs:


W(G | x)        = (tau | True)    if (x:tau) in G
W(G | x)        = Fail           otherwise
W(G | (e1, e2)) = ((tau1,tau2 | C1/\C2)
                 where
                   W(G | e1)= (tau1,C1)
                   W(G | e2)= (tau2,C2)