reserve S for Subset of TOP-REAL 2,
  C,C1,C2 for non empty compact Subset of TOP-REAL 2,
  p,q for Point of TOP-REAL 2;
reserve i,j,k for Nat,
  t,r1,r2,s1,s2 for Real;
reserve D1 for non vertical non empty compact Subset of TOP-REAL 2,
  D2 for non horizontal non empty compact Subset of TOP-REAL 2,
  D for non vertical non horizontal non empty compact Subset of TOP-REAL 2;

theorem
  (SpStSeq S)/.5 = NW-corner S
proof
  set g = <*NW-corner S,NE-corner S,SE-corner S*>;
  2 in {1,2} by TARSKI:def 2;
  then
A1: 2 in dom <*SW-corner S,NW-corner S*> by FINSEQ_1:2,89;
  len g = 3 by FINSEQ_1:45;
  hence (SpStSeq S)/.5 = (SpStSeq S)/.(len g + 2)
    .= <*SW-corner S,NW-corner S*>/.2 by A1,FINSEQ_4:69
    .= NW-corner S by FINSEQ_4:17;
end;
