theorem Th32:
  X c= Rank A iff X in Rank succ A
proof
  thus X c= Rank A implies X in Rank succ A
  proof
    assume X c= Rank A;
then  X in bool Rank A;
    hence thesis by Lm2;
  end;
  assume X in Rank succ A;
then  X in bool Rank A by Lm2;
  hence thesis;
end;
