docs: def add_field(self, name, ftype, docfield=None):
          """ Add a field to the document (and to the underlying schema)
          
          :param name: name of the new field
          :type name: str
          :param ftype: type of the new field
          :type ftype: subclass of :class:`.GenericType`
          """
          self.schema.add_field(name, ftype)
          self[name] = docfield or DocField.FromType(ftype)