code: def _get_epoch(_str):
      """Take incoming string, return its epoch."""
      _return = None
      if _str.startswith("A.D. "):
          _return = "ad"
      elif _str.startswith("a. A.D. "):
          _return = None  # ?
      elif _str.startswith("p. A.D. "):
          _return = "ad"
      elif regex.match(r"^[0-9]+ B\.C\. *", _str):
          _return = "bc"
      elif regex.match(r"^a\. *[0-9]+ B\.C\. *", _str):
          _return = "bc"
      elif regex.match(r"^p\. *[0-9]+ B\.C\. *", _str):
          _return = None  # ?
      elif _str == "Incertum" or _str == "Varia":
          _return = _str
      return _return
  
imports: import json
  import os
  import regex
  from cltk.corpora.grc.tlg.author_date import AUTHOR_DATE
  from cltk.corpora.grc.tlg.author_epithet import AUTHOR_EPITHET
  from cltk.corpora.grc.tlg.author_female import AUTHOR_FEMALE
  from cltk.corpora.grc.tlg.author_geo import AUTHOR_GEO
  from cltk.corpora.grc.tlg.id_author import ID_AUTHOR
  from cltk.corpora.grc.tlg.index_lists import INDEX_LIST
  from cltk.corpora.grc.tlg.work_numbers import WORK_NUMBERS
label_desc: Use image node_id 19 for calling a global function with example usage: _get_epoch(_str) and returns: _return