How to add xs:annotation inside the xs:choice

I'm trying to add the annotation element inside the xs:choice. According to the xs:choice syntax, this could be possible. I could not find the sample like this, so my current version of xsd file contains an element:

<xs:complexType>
            <xs:sequence>               
                <xs:choice>
                    <xs:annotation>
                        <xs:appinfo>
                            <flex:ControlHeadline>Control Headline of the Choice-Element</flex:ControlHeadline>
                            <flex:helpText>Help Text of The Choice-Element</flex:helpText>
                        </xs:appinfo>
                    </xs:annotation>

                    <xs:element name="Some_Boolean1" type="xs:boolean" minOccurs="1" maxOccurs="1" default="true">

                    <xs:element name="Some_String1" type="xs:string" minOccurs="1" maxOccurs="1"></xs:element>

                </xs:choice>                
            </xs:sequence>
        </xs:complexType>

However, while parsing the xsd file, the Annotation of the object 'System.Xml.Schema.XmlSchemaChoice' is always null. Could somebody give some working sample or add some hints. Any help would be appreciated. Thanks.