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homomorphism of polynomial rings
PolyRingHom(R: RING, S: RING, CoeffHom: RINGHOM, images: LIST): RINGHOM
This function create the homomorphism of (polynomial) algebras between
R and S.
This is uniquely defined by the images of the indeterminated of R and
the homomorphism CoeffRing(R) into S.
/**/ R ::= QQ[x,y];
/**/ S ::= QQ[a,b,c];
/**/ SmodJ := NewQuotientRing(S, ideal(RingElem(S,"a")^2-1));
/**/ Use SmodJ;
/**/ phi := PolyRingHom(R, SmodJ, CanonicalHom(QQ,SmodJ), [a,b]);
/**/ Use R;