XSD 1.1 assertions are a great mistery for me

Hello !

I want to define a complex type that represents a parameter.
A parameter MUST have a @name
A parameter MAY have a @as
A parameter has either a @value, or a @abstract
A parameter that has a @abstract MUST have @as

I've defined this :

<xs:schema
   xmlns:xs="http://www.w3.org/2001/XMLSchema"
   xmlns:vc="http://www.w3.org/2007/XMLSchema-versioning"
   elementFormDefault="qualified"
   attributeFormDefault="unqualified"
   vc:minVersion="1.1">

   <xs:element name="data">
     <xs:complexType>
       <xs:sequence maxOccurs="unbounded">
         <xs:element name="param" type="TParam"/>
       </xs:sequence>
     </xs:complexType>
   </xs:element>

   <xs:complexType name="TParam">
     <xs:attribute name="name" type="xs:string" use="required"/>
     <xs:attribute name="as" type="xs:string" use="optional"/>
     <xs:attribute name="value" type="xs:string" use="optional"/>
     <xs:attribute name="abstract" fixed="true" use="optional" />
     <xs:assert test="count(@value | @abstract) eq 1" id="assert1"/>
     <xs:assert test="if(exists(@abstract)) then exists(@as) else true()" 
id="assert2"/>
   </xs:complexType>
</xs:schema>

And this file :

<data xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:noNamespaceSchemaLocation="param.xsd">
   <param name="p1" />                               <!-- should fail 
assert1 -->
   <param name="p2" value="v2"/>                     <!-- should be valid 
-->
   <param name="p3" value="v3" as="xs:string"/>      <!-- should be valid 
-->
   <param name="p4" as="xs:string" abstract="true"/> <!-- should be valid 
-->
   <param name="p5" abstract="true"/>                <!-- should fail 
assert2 -->
   <param name="p6" abstract="true" value="v6"/>     <!-- should fail 
assert2 -->
</data>

All param except p4 are invalid.
p1 fails assert2
p2 fails assert1 & assert2
p3 fails assert2
p4 is valid
p5 fails assert2
p6 fails assert1 & assert2

I do not know what I miss... Any help will be much appreciated !

Best,
Christophe