
theorem Th3:
  for R being 1-sorted,
      X being Subset of R holds
    { x where x is Element of R : {} meets X } = {}R
  proof
    let R be 1-sorted;
    let X be Subset of R;
    thus {x where x is Element of R : {} meets X} c= {}R
    proof
      let y be object;
      assume y in {x where x is Element of R : {} meets X}; then
      ex z being Element of R st z = y & {} meets X;
      then {} /\ X <> {};
      hence thesis;
    end;
    thus thesis;
  end;
