<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _add_message_field(self, field_name, value, params):
 """Converts a . delimitied field name to a message field in parameters. This adds the field to the params dict, broken out so that message parameters appear as sub-dicts within the outer param. For example: {'a.b.c': ['foo']} becomes: {'a': {'b': {'c': ['foo']}}} Args: field_name: A string containing the '.' delimitied name to be converted into a dictionary. value: The value to be set. params: The dictionary holding all the parameters, where the value is eventually set. """

    if '.' not in field_name:
      params[field_name] = value
      return

    root, remaining = field_name.split('.', 1)
    sub_params = params.setdefault(root, {})
    self._add_message_field(remaining, value, sub_params)