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

theorem Th9:
  x in A^s iff x in A & U_FT x \ {x} misses A
proof
  thus x in A^s implies x in A & U_FT x \ {x} misses A
  proof
    assume x in A^s;
    then ex y st y = x & y in A & U_FT y \ {y} misses A;
    hence thesis;
  end;
  assume x in A & U_FT x \ {x} misses A;
  hence thesis;
end;
