
theorem
  for C being non empty category,
      a,b be Object of C, f being Morphism of a,b
  st f is isomorphism holds f is monomorphism & f is epimorphism
  proof
    let C be non empty category;
    let a,b be Object of C;
    let f be Morphism of a,b;
    assume
A1: f is isomorphism;
    f is section_ by A1;
    hence f is monomorphism by Th28;
    f is retraction by A1;
    hence f is epimorphism by Th32;
  end;
