reserve G for Go-board,
  i,j,k,m,n for Nat;

theorem Th19:
  for f being non trivial FinSequence of TOP-REAL 2 holds LSeg(f,1 ) = L~(f|2)
proof
  let f be non trivial FinSequence of TOP-REAL 2;
A1: 1+1 <= len f by NAT_D:60;
  then
A2: f|2 = <*f/.1,f/.2*> by FINSEQ_5:81;
  thus LSeg(f,1) = LSeg(f/.1,f/.(1+1)) by A1,TOPREAL1:def 3
    .= L~(f|2) by A2,SPPOL_2:21;
end;
