docs: def tokens(istr):
      """
      Same as tokenize, but returns only tokens
      (and at all parantheses levels).
      """
  
      # make a list of all alphanumeric tokens
      toks = re.findall(r'[^\*\\\+\-\^\(\)]+\(?', istr)
  
      # remove the functions
      return [t for t in toks if not t.endswith('(')]