new_contents: __all__ = ['SUB']
  
  from .base import NaryHead, Head, heads_precedence, ArithmeticHead
  
  class SubHead(ArithmeticHead, NaryHead):
  
      """
      SubHead represents subtraction n-ary operation where operands is
      given as a n-tuple of expressions.
      """
      precedence = Head.precedence_map['SUB']
      op_mth = '__sub__'
      op_rmth = '__rsub__'
      op_symbol = ' - '
      def __repr__(self): return 'SUB'
  
      def data_to_str_and_precedence(self, cls, sub_list):
          num_p = heads_precedence.NUMBER
          m = len(sub_list)
          if not m:
              return '0', num_p
          add_p = heads_precedence.ADD
          r = ''
          for term in sub_list:
              h, d = term.pair
              t,t_p = h.data_to_str_and_precedence(cls, d)
              if m>1:
                  if not r:
                      r += '(' + t + ')' if t_p < add_p else t
                  elif t.startswith('-'):
                      r += ' + ' + t[1:]
                  else:
                      r += ' - (' + t + ')' if t_p < add_p else ' - ' + t
              else:
                  return t, t_p
          return r, add_p
  
  
  SUB = SubHead()
  
subject: Add missing file.