<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_schema(self, schema):
 """ Merges in an existing schema. arguments: * `schema` (required - `dict` or `SchemaNode`):
 an existing JSON Schema to merge. """


        # serialize instances of SchemaNode before parsing
        if isinstance(schema, SchemaNode):
            schema = schema.to_schema()

        for subschema in self._get_subschemas(schema):
            # delegate to SchemaType object
            schema_generator = self._get_generator_for_schema(subschema)
            schema_generator.add_schema(subschema)

        # return self for easy method chaining
        return self