class_docstr: Suggested backoff chain; includes at least on of each
  type of major sequential backoff class from backoff.py.
code: def _define_lemmatizer(self):
      # Suggested backoff chain--should be tested for optimal order
      self.backoff0 = None
      self.backoff1 = IdentityLemmatizer(verbose=self.VERBOSE)
      self.backoff2 = DictLemmatizer(
          lemmas=self.GREEK_OLD_MODEL,
          source="Morpheus Lemmas",
          backoff=self.backoff1,
          verbose=self.VERBOSE,
      )
      self.backoff3 = RegexpLemmatizer(
          self.greek_sub_patterns,
          source="CLTK Greek Regex Patterns",
          backoff=self.backoff2,
          verbose=self.VERBOSE,
      )
      self.backoff4 = UnigramLemmatizer(
          self.train_sents,
          source="CLTK Sentence Training Data",
          backoff=self.backoff3,
          verbose=self.VERBOSE,
      )
      self.backoff5 = DictLemmatizer(
          lemmas=self.GREEK_MODEL,
          source="Greek Model",
          backoff=self.backoff4,
          verbose=self.VERBOSE,
      )
      self.lemmatizer = self.backoff5
  
file_path: cltk/src/cltk/lemmatize/grc.py
imports: import os
  from typing import List
  from cltk.lemmatize.backoff import DictLemmatizer, IdentityLemmatizer, RegexpLemmatizer, UnigramLemmatizer
  from cltk.utils import CLTK_DATA_DIR
  from cltk.utils.file_operations import open_pickle
label_desc: Use image node_id 2 for calling the GreekBackoffLemmatizer obj's underlying member method code with example usage: obj._define_lemmatizer() without return types