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

theorem Th5:
  x in A^delta iff U_FT x meets A & U_FT x meets A`
proof
  thus x in A^delta implies U_FT x meets A & U_FT x meets A`
  proof
    assume x in A^delta;
    then ex y st y=x & U_FT y meets A & U_FT y meets A`;
    hence thesis;
  end;
  assume U_FT x meets A & U_FT x meets A`;
  hence thesis;
end;
