theorem
  F1 is_naturally_transformable_to F2 implies (id id B)(#)s = s
proof
  assume
A1: F1 is_naturally_transformable_to F2;
  then
A2: (id B)*F1 is_naturally_transformable_to (id B)*F2 by Th20;
  thus (id id B)(#)s = (id((id B)*F2))`*`((id B)*s) by Th30
    .= (id B)*s by A2,NATTRA_1:24
    .= s by A1,Th33;
end;
