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;
