def _get_global_entities(nest):
  """Returns list of string indices representing each global entity.

  For example, if 'global_entities/0' and 'global_entities/1' exist in the
  input nest, this function will return the list ['0', '1'].

  Args:
    nest: A tensor or spec nest generated from a falken observation spec.
  Returns:
    A list of strings, representing the indices / IDs of each global entity.
  """
  entities = nest.get('global_entities', None)
  if not entities:
    return []
  return list(entities.keys())