docs: def vectorizable_features(fcs):
      '''Discovers the ordered set of vectorizable features in ``fcs``.
  
      Returns a list of feature names, sorted lexicographically.
      Feature names are only included if the corresponding
      features are vectorizable (i.e., they are an instance of
      :class:`collections.Mapping`).
      '''
      is_mapping = lambda obj: isinstance(obj, collections.Mapping)
      return sorted(set([name for fc in fcs for name in fc if is_mapping(fc[name])]))