theorem
  F1 is_naturally_transformable_to F2 implies G*s = (id G)(#)s
proof
  assume F1 is_naturally_transformable_to F2;
  then G*F1 is_naturally_transformable_to G*F2 by Th20;
  hence G*s = id(G*F2)`*`(G*s) by NATTRA_1:24
    .= (id G)(#)s by Th30;
end;
