reserve x,y for Real,
  u,v,w for set,
  r for positive Real;

theorem Th18:
  for a,b being Real holds |[a,b]| in y>=0-plane iff b >= 0
proof
  let a,b be Real;
  |[a,b]| in y>=0-plane iff ex x,y st |[a,b]| = |[x,y]| & y >= 0;
  hence thesis by SPPOL_2:1;
end;
