code: def _clauses(lexer, varname, nvars):
      
      tok = next(lexer)
      toktype = type(tok)
      if toktype is OP_not or toktype is IntegerToken:
          lexer.unpop_token(tok)
          first = _clause(lexer, varname, nvars)
          rest = _clauses(lexer, varname, nvars)
          return (first, ) + rest
      # null
      else:
          lexer.unpop_token(tok)
          return tuple()
docs: Return a tuple of DIMACS CNF clauses.