let empty x = match x with
  | '1'->['-'] | '2'->['-';'-']| '3'->['-';'-';'-']| '4'->['-';'-';'-';'-']
  | '5'->['-';'-';'-';'-';'-']|'6'-> ['-';'-';'-';'-';'-';'-']|'7'->['-';'-';'-';'-';'-';'-';'-'] 
  |'8'-> ['-';'-';'-';'-';'-';'-';'-';'-'] 
  |'r'->['r']|'b'->['b']|'n'->['n']|'q'->['q']|'k'->['k']|'p'->['p']
  |'R'->['R']|'B'->['B']|'N'->['N']|'Q'->['Q']|'K'->['K']|'P'->['P']
  |_->[]