theorem
  X1,X2 are_isomorphic & X2,X3 are_isomorphic implies X1,X3 are_isomorphic
  proof
    given F1,G1 such that
A1: F1,G1 form_isomorphism_between X1,X2;
    given F2,G2 such that
A2: F2,G2 form_isomorphism_between X2,X3;
    take F2*F1, G2*G1;
    thus thesis by A1,A2,Th44;
  end;
