code: def get_bibtex(identifier):
      
      identifier_type, identifier_id = identifier
      if identifier_type not in __valid_identifiers__:
          return None
  
      # Dynamically call the ``get_bibtex`` method from the associated module.
      module = sys.modules.get("libbmc.%s" % (identifier_type,), None)
      if module is None:
          return None
      return getattr(module, "get_bibtex")(identifier_id)
docs: Try to fetch BibTeX from a found identifier.
  
      .. note::
  
          Calls the functions in the respective identifiers module.
  
      :param identifier: a tuple (type, identifier) with a valid type.
      :returns: A BibTeX string or ``None`` if an error occurred.
      # TODO: Should return a BiBTeX object?