theorem Th38:
  (SpStSeq S)/.4 = SW-corner S
proof
  set g = <*NW-corner S,NE-corner S,SE-corner S*>;
  1 in {1,2} by TARSKI:def 2;
  then
A1: 1 in dom <*SW-corner S,NW-corner S*> by FINSEQ_1:2,89;
  len g = 3 by FINSEQ_1:45;
  hence (SpStSeq S)/.4 = (SpStSeq S)/.(len g + 1)
    .= <*SW-corner S,NW-corner S*>/.1 by A1,FINSEQ_4:69
    .= SW-corner S by FINSEQ_4:17;
end;
