
theorem Th5:
  for C being category, o1, o2 being Object of C, A being Morphism
  of o1,o2 st A is iso holds A is retraction coretraction
proof
  let C be category, o1, o2 be Object of C, A be Morphism of o1,o2;
  assume
A1: A is iso;
  then A * A" = idm o2;
  then A" is_right_inverse_of A;
  hence A is retraction;
  A" * A = idm o1 by A1;
  then A" is_left_inverse_of A;
  hence thesis;
end;
