theorem Th42:
  R,S are_isomorphic & S,T are_isomorphic implies R,T are_isomorphic
proof
  given F such that
A1: F is_isomorphism_of R,S;
  given G such that
A2: G is_isomorphism_of S,T;
  take G*F;
  thus thesis by A1,A2,Th41;
end;
