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

theorem
  for f being non empty FinSequence for g being FinSequence holds len g
  <= len(f^'g)
proof
  let f be non empty FinSequence, g be FinSequence;
  per cases;
  suppose
    g = {};
    hence thesis;
  end;
  suppose
A1: g <> {};
A2: len f >= 1 by NAT_1:14;
    len (f^'g) +1 = len f + len g by A1,FINSEQ_6:139;
    then len (f^'g) +1 >= 1 + len g by A2,XREAL_1:6;
    hence thesis by XREAL_1:6;
  end;
end;
