reserve x,y,z,X for set,
  T for Universe;

theorem Th20:
  for S being non empty 1-sorted, N being net of S holds N
  is_eventually_in the carrier of S
proof
  let S be non empty 1-sorted, N be net of S;
  set i = the Element of N;
  take i;
  let j be Element of N;
  assume i <= j;
  thus thesis;
end;
