class_docstr: Base class for Filter objects. A Filter subclass must implement a
  filter() method that takes a single argument, which is an iterator of Token
  objects, and yield a series of Token objects in return.
  
  Filters that do morphological transformation of tokens (e.g. stemming)
  should set their ``is_morph`` attribute to True.
class_docstr_tok: ["Base","class","for","Filter","objects",".","A","Filter","subclass","must","implement","a","filter","(",")","method","that","takes","a","single","argument",",","which","is","an","iterator","of","Token","objects",",","and","yield","a","series","of","Token","objects","in","return",".","Filters","that","do","morphological","transformation","of","tokens","(","e.g",".","stemming",")","should","set","their","``","is_morph","``","attribute","to","True","."]
code: def __call__(self, tokens):
      raise NotImplementedError
  
imports: from itertools import chain
  from whoosh.compat import next
  from whoosh.analysis.acore import Composable
  from whoosh.util.text import rcompile
label_desc: Use image node_id 3 for calling the Filter obj's underlying member method code with example usage: obj.__call__(tokens) without return types