Using proper XPATH in assert

I am having problem when validating the following XML Schema file. It shows me that the xpath is not valid. In the assert test I want to check if the value of BOID=OID then A_Membership_Degree is greater or equal to B_Membership_Degree.

<xs:schema xmlns:xs="http://ift.tt/tphNwY" 
           xmlns:vc="http://ift.tt/REsXlC" 
           elementFormDefault="qualified" 
           attributeFormDefault="unqualified" 
           vc:minVersion="1.1">
  <xs:element name="Document">
    <xs:complexType>
      <xs:sequence maxOccurs="unbounded">
        <xs:element ref="A" minOccurs="0" maxOccurs="unbounded"/>
        <xs:element ref="B" minOccurs="0" maxOccurs="unbounded"/>
        <xs:element ref="C" minOccurs="0" maxOccurs="unbounded"/>
      </xs:sequence>
      <xs:assert 
        test="if(.//@OID eq  .//@BOID) 
              then ./A/@A_Membership_Degree  
                   ge ./B/@A_Membership_Degree"/>
    </xs:complexType>
    <xs:key name="aKey">
      <xs:selector xpath="A"/>
      <xs:field xpath="@OID"/>
    </xs:key>
    <xs:keyref name="aKeyRef" refer="aKey">
      <xs:selector xpath="./B"/>
      <xs:field xpath="@BOID"/>
    </xs:keyref>
    <xs:keyref name="aKeyRef1" refer="aKey">
      <xs:selector xpath="./C"/>
      <xs:field xpath="@COID"/>
    </xs:keyref>
  </xs:element>
  <xs:element name="A" type="Atype"/>
  <xs:complexType name="Atype">
    <xs:sequence minOccurs="0" maxOccurs="unbounded">
      <xs:element name="A1" type="xs:string" minOccurs="1" maxOccurs="1"/>
      <xs:element name="A2" type="xs:string" minOccurs="1" maxOccurs="1"/>
    </xs:sequence>
    <xs:attribute name="OID" type="xs:string" use="required"/>
    <xs:attribute name="A_Membership_Degree" type="FuzzyValue" use="optional"/>
    <xs:attribute name="D_Membership_Degree" type="FuzzyValue" use="optional"/>
  </xs:complexType>
  <xs:element name="B" type="Btype"/>
  <xs:complexType name="Btype">
    <xs:sequence>
      <xs:element name="B1" type="xs:string" minOccurs="1" maxOccurs="1"/>
      <xs:element name="B2" type="xs:string" minOccurs="1" maxOccurs="1"/>
    </xs:sequence>
    <xs:attribute name="BOID" type="xs:string" use="required"/>
    <xs:attribute name="B_Membership_Degree" type="FuzzyValue" use="optional"/>
  </xs:complexType>
  <xs:element name="C" type="Ctype"/>
  <xs:complexType name="Ctype">
    <xs:sequence>
      <xs:element name="C1" type="xs:string" minOccurs="1" maxOccurs="1"/>
      <xs:element name="C2" type="xs:string" minOccurs="1" maxOccurs="1"/>
    </xs:sequence>
    <xs:attribute name="COID" type="xs:string" use="required"/>
    <xs:attribute name="C_Membership_Degree" type="FuzzyValue" use="required"/>
  </xs:complexType>
  <xs:simpleType name="FuzzyValue">
    <xs:restriction base="xs:decimal">
      <xs:minInclusive value="0"/>
      <xs:maxInclusive value="1"/>
    </xs:restriction>
  </xs:simpleType>
</xs:schema>

Thanks very much for your help Reema