Hi Ravi:

negation adds real expressive power by making strategies more generic:

![A,C] = merge([A,!C],[!A,C],[!A,!C])

Without negation, we would have to explicitely enumerate the nodes different from C, etc.:

![A,C] = merge([A,{R,S,T}],[{G,H,I},C],[{G,H,I},{R,S,T}])

In this sense, negation is essential.

-- Karl