theorem Th21:
  x in A^Foi iff ex V st V in U_FMT x & V c= A
proof
  thus x in A^Foi implies ex V st V in U_FMT x & V c= A
  proof
    assume x in A^Foi;
    then ex y st y=x & ex V st V in U_FMT y & V c= A;
    hence thesis;
  end;
  assume ex V st V in U_FMT x & V c= A;
  hence thesis;
end;
