reserve o,m for set;
reserve C for Cartesian_category;
reserve a,b,c,d,e,s for Object of C;

theorem Th28:
  Hom(a,a[x][1]C) <> {} & Hom(a,[1]C[x]a) <> {}
proof
  Hom(a,[1]C) <> {} & Hom(a,a) <> {} by Th13;
  hence thesis by Th23;
end;
