
theorem Th1:
  for R being total non empty RelStr,
      x being Element of R holds
    x in field the InternalRel of R
  proof
    let R be total non empty RelStr;
    let x be Element of R;
    dom the InternalRel of R = the carrier of R by PARTFUN1:def 2;
    hence thesis by XBOOLE_0:def 3;
  end;
