theorem
  a,b are_isomorphic & b,c are_isomorphic implies a,c are_isomorphic
proof
  given f being Morphism of a,b such that
A1: f is invertible;
  given g being Morphism of b,c such that
A2: g is invertible;
  take g*f;
  thus thesis by A1,A2,Th40;
end;
