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

theorem Th19:
  y=0-line c= y>=0-plane
proof
  let x be object;
  assume x in y=0-line;
  then reconsider x as Element of y=0-line;
A1: x = |[x`1,x`2]| by EUCLID:53;
  then x`2 = 0 by Th15;
  hence thesis by A1;
end;
