检查XPath中属性的默认值

i have XML schema:

我有XML模式:

<xsd:complexType name="contactsType">
  <xsd:sequence>
    <xsd:element name="contact" type="contactType" minOccurs="0" maxOccurs="unbounded"/>
  </xsd:sequence>
  <xsd:attribute name="visible" type="xsd:boolean" default="true"/>
</xsd:complexType>
<xsd:complexTyp