theorem Th40:
  len SpStSeq S = 5
proof
  thus len SpStSeq S = len<*NW-corner S,NE-corner S,SE-corner S*> + len<*
  SW-corner S,NW-corner S*> by FINSEQ_1:22
    .= len<*SW-corner S,NW-corner S*> + 3 by FINSEQ_1:45
    .= 2 + 3 by FINSEQ_1:44
    .= 5;
end;
