code: def _dict_to_pio(d, class_=None):
      
      d = keys_to_snake_case(d)
      if class_:
          return class_(**d)
      if 'category' not in d:
          raise ValueError('Dictionary does not contains a category field: ' + ', '.join(d.keys()))
      elif d['category'] == 'system':
          return System(**d)
      elif d['category'] == 'system.chemical':
          return ChemicalSystem(**d)
      elif d['category'] == 'system.chemical.alloy':  # Legacy support
          return Alloy(**d)
      elif d['category'] == 'system.chemical.alloy.phase':  # Legacy support
          return ChemicalSystem(**d)
      raise ValueError('Dictionary does not contain a valid top-level category: ' + str(d['category']))
docs: Convert a single dictionary object to a Physical Information Object.
  
      :param d: Dictionary to convert.
      :param class_: Subclass of :class:`.Pio` to produce, if not unambiguous
      :return: Single object derived from :class:`.Pio`.