
theorem Th17:
  for L being non empty RelStr, R being RelStr for x,y being
  Element of ClosureSystems L st y = R holds x <= y iff x is SubRelStr of R
proof
  let L be non empty RelStr, R be RelStr;
  let x,y be Element of ClosureSystems L;
  reconsider a = x, b = y as Element of Sub L by YELLOW_0:58;
  x <= y iff a <= b by YELLOW_0:59,60;
  hence thesis by Th15;
end;
