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

theorem Th26:
  y=0-line is closed Subset of Niemytzki-plane
proof
  reconsider B = y=0-line as Subset of Niemytzki-plane by Def3,Th19;
  reconsider A = y>=0-plane \ y=0-line as open Subset of Niemytzki-plane by
Th25;
  B` = A by Def3;
  then A` = y=0-line;
  hence thesis;
end;
