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

theorem Th15:
  for a,b being Real holds |[a,b]| in y=0-line iff b = 0
proof
  let a,b be Real;
  |[a,b]| in y=0-line iff ex x st |[a,b]| = |[x,0]|;
  hence thesis by SPPOL_2:1;
end;
