reconsider A = y=0-line as Subset of Niemytzki-plane by Def3, Th23;
A is nowhere_dense

proof

thus Int (Cl A) = Int A by Th30, PRE_TOPC:52
.= {} by Th39 ; :: according to TOPS_3:def 3 :: thesis: verum

end;

hence y=0-line is nowhere_dense Subset of Niemytzki-plane ; :: thesis: verum