theorem Th48:
  for x being Point of T, cB being basis of BOOL2F NeighborhoodSystem x holds
  x in lim_filter(s,<. Frechet_Filter(NAT),Frechet_Filter(NAT).)) iff
  for B being Element of cB holds ex n being Nat st square-uparrow n c= s"(B)
  proof
    let x be Point of T,cB be basis of BOOL2F NeighborhoodSystem x;
    hereby
      assume
A1:   x in lim_filter(s,<. Frechet_Filter(NAT),Frechet_Filter(NAT).) );
      hereby
        let B be Element of cB;
        B is a_neighborhood of x by YELLOW19:2;
        hence ex n being Nat st square-uparrow n c= s"(B) by A1,Th47;
      end;
    end;
    assume
A2: for B being Element of cB holds ex n being Nat st
      square-uparrow n c= s"(B);
    now
      let A be a_neighborhood of x;
A3:   A is Element of BOOL2F NeighborhoodSystem x by YELLOW19:2;
      cB is filter_basis;
      then consider B be Element of cB such that
A4:   B c= A by A3;
A5:   ex n being Nat st square-uparrow n c= s"(B) by A2;
      s"(B) c= s"(A) by A4,RELAT_1:145;
      hence ex n being Nat st square-uparrow n c= s"(A) by A5,XBOOLE_1:1;
    end;
    hence x in lim_filter(s,<. Frechet_Filter(NAT),Frechet_Filter(NAT).) )
      by Th47;
  end;
