reserve FT for non empty RelStr;
reserve x, y, z for Element of FT;
reserve A for Subset of FT;

theorem Th8:
  x in A^b iff U_FT x meets A
proof
  thus x in A^b implies U_FT x meets A
  proof
    assume x in A^b;
    then ex y st y = x & U_FT y meets A;
    hence thesis;
  end;
  assume U_FT x meets A;
  hence thesis;
end;
