theorem Th66:
  for G,H being Group holds G,H are_isomorphic implies
  H,G are_isomorphic
proof
  let G,H be Group;
  assume G,H are_isomorphic;
  then consider h being Homomorphism of G,H such that
A1: h is bijective;
  reconsider g = h" as Homomorphism of H,G by A1,Th62;
  take g;
  thus thesis by A1,Th63;
end;
