theorem
  f is coretraction & g is coretraction implies g*f is coretraction
proof
  assume
A1: Hom(a,b) <> {} & Hom(b,a) <> {};
  given i being Morphism of b,a such that
A2: i*f = id a;
  assume
A3: Hom(b,c) <> {} & Hom(c,b) <> {};
  given j being Morphism of c,b such that
A4: j*g = id b;
  thus
A5: Hom(a,c) <> {} & Hom(c,a) <> {} by A1,A3,CAT_1:24;
  take i*j;
  thus i*j*(g*f) = i*(j*(g*f)) by A1,A3,A5,CAT_1:25
    .= i*(j*g*f) by A1,A3,CAT_1:25
    .= id a by A1,A2,A4,CAT_1:28;
end;
