theorem
  id b is monic
proof
A1: now
    let a;
    let f1,f2 be Morphism of a,b;
    assume
A2: Hom(a,b)<>{};
    then (id b)*f1 = f1 by Th23;
    hence (id b)*f1 = (id b)*f2 implies f1 = f2 by A2,Th23;
  end;
  thus thesis by A1;
end;
