theorem
  G,H are_isomorphic & H,I are_isomorphic implies G,I are_isomorphic
proof
  assume that
A1: G,H are_isomorphic and
A2: H,I are_isomorphic;
  consider g such that
A3: g is bijective by A1;
  consider h1 such that
A4: h1 is bijective by A2;
  (h1 * g) is bijective by A3,A4,Th64;
  hence thesis;
end;
