reserve n for Element of NAT;
reserve i for Integer;
reserve G,H,I for Group;
reserve A,B for Subgroup of G;
reserve N for normal Subgroup of G;
reserve a,a1,a2,a3,b,b1 for Element of G;
reserve c,d for Element of H;
reserve f for Function of the carrier of G, the carrier of H;
reserve x,y,y1,y2,z for set;
reserve A1,A2 for Subset of G;
reserve N for normal Subgroup of G;
reserve S,T1,T2 for Element of G./.N;
reserve g,h for Homomorphism of G,H;
reserve h1 for Homomorphism of H,I;

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;
