docstr: Returns the word indices in current_text which are able to be
  modified.
docstr_tok: ["Returns","the","word","indices","in","current_text","which","are","able","to","be","modified","."]
code: def _get_modifiable_indices(self, current_text):
      """Returns the word indices in current_text which are able to be
      modified."""
  
      if (
          len(current_text.attack_attrs["modified_indices"])
          >= self.max_num_words
      ):
          return set()
      else:
          return set(range(len(current_text.words)))
  
imports: from textattack.constraints import PreTransformationConstraint
label_desc: Use image node_id 2 for calling the MaxNumWordsModified obj's underlying member method code with example usage: obj._get_modifiable_indices(current_text) and returns: set, set