theorem
  id b is epi
proof
A1: now
    let c;
    let g1,g2 be Morphism of b,c;
    assume
A2: Hom(b,c)<>{};
    then g1*(id b) = g1 by Th24;
    hence g1*(id b) = g2*(id b) implies g1 = g2 by A2,Th24;
  end;
  thus thesis by A1;
end;
