firma-ns.xsd:18 element complexType: Schemas parser error : Element '{http://www.w3.org/2001/XMLSchema}element': The content is not valid. Expected is (annotation?, ((simpleType | complexType)?, (unique | key | keyref)*)).
firma-ns.xsd:33 element keyref: Schemas parser error : Element '{http://www.w3.org/2001/XMLSchema}keyref', attribute 'refer': References from this schema to components in no namespace are not allowed, since not indicated by an import statement.
firma-ns.xsd:41: element complexType: Schemas parser error : Element '{http://www.w3.org/2001/XMLSchema}element': The content is not valid. Expected is (annotation?, ((simpleType | complexType)?, (unique | key | keyref)*)).
WXS schema firma-ns.xsd failed to compile

请问上面的错误应该怎么修改呢?