code: def _get_ncbi_db_url():
      
  
      if "NCBI_DB_URL" in os.environ:
          return os.environ["NCBI_DB_URL"]
  
      if "_NCBI_URL_KEY" in os.environ:
          url_key = os.environ["_NCBI_URL_KEY"]
      else:
          sdlc = _stage_from_version(hgvs.__version__)
          url_key = "public_{sdlc}".format(sdlc=sdlc)
      return hgvs.global_config['NCBI'][url_key]
docs: returns NCBI DB URL based on environment variables and code version
  
      * if NCBI_DB_URL is set, use that
      * Otherwise, if _NCBI_URL_KEY is set, use that as the name of a
        config file entry and use the corresponding URL
      * Otherwise,