reserve C for CatStr;
reserve f,g for Morphism of C;
reserve C for non void non empty CatStr,
  f,g for Morphism of C,
  a,b,c,d for Object of C;
reserve o,m for set;
reserve B,C,D for Category;
reserve a,b,c,d for Object of C;
reserve f,f1,f2,g,g1,g2 for Morphism of C;
reserve f,f1,f2 for Morphism of a,b;
reserve f9 for Morphism of b,a;
reserve g for Morphism of b,c;
reserve h,h1,h2 for Morphism of c,d;

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;
