Unification

gcd(u, 0, u)
gcd(u, v, w) <- not zero(v), gcd(v, u mod v, w)

<-gcd(29, 7, X)

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gcd(29, 7, X) <- not zero(7), gcd(7,1,X), gcd(29, 7, X)
gcd(1, 7, X) <- not zero(7), gcd(7,0,X), gcd(1, 7, X)
gcd(1,0,1) <- gcd(1,0,1)