Add a schema to the list of definition

        :param id: id of the schema.
        :param schema: the schema as a dict or a
                       :class:schemabuilder.primitives.Generic
        :return: reference to schema.
        :rtype: :class:`schemabuilder.schema.Ref`

def define(self, id, schema):
        """Add a schema to the list of definition

        :param id: id of the schema.
        :param schema: the schema as a dict or a
                       :class:schemabuilder.primitives.Generic
        :return: reference to schema.
        :rtype: :class:`schemabuilder.schema.Ref`

        """
        self.definitions[id] = schema
        self._schema = None
        return self.ref(id)