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2.10.2 Composition of RINGHOM
Two RINGHOM phi: R-->S and psi: S-->T can be composed.

Example
/**/  R := NewPolyRing(QQ, "a");
/**/  S := NewFractionField(R);  -- QQ(a)
/**/  T := NewPolyRing(S, "x");
/**/  phi := CanonicalHom(R,S);
/**/  psi := CanonicalHom(S,T);
/**/  theta := psi(phi);
/**/  theta(ReadExpr(R, "a^2+a-1"));
a^2 +a -1
/**/  RingOf(theta(ReadExpr(R, "a^2+a-1"))) = T;
true