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

theorem Th7:
  x in A^i iff U_FT x c= A
proof
  thus x in A^i implies U_FT x c= A
  proof
    assume x in A^i;
    then ex y st y=x & U_FT y c= A;
    hence thesis;
  end;
  assume U_FT x c= A;
  hence thesis;
end;
