code: def first_field(self):
          
          for name, item in enumerate(self):
              # Container
              if is_container(item):
                  field = item.first_field()
                  # Container is not empty
                  if field is not None:
                      return field
              # Field
              elif is_field(item):
                  return item
              else:
                  raise MemberTypeError(self, item, name)
          return None
docs: Returns the first :class:`Field` in the `Sequence` or ``None``
          for an empty `Sequence`.