prohibited attributes in an attributeGroup

My question is regarding having prohibited attributes in an
attributeGroup which is referenced from a complex type derived from
restriction from a base type which has the same attribute as optional.

Base Type:

<xsd:complexType name="base">
	<xsd:attribute name="a" />
</xsd:complexType>

Derived Type:

<xsd:complexType name="derived">
  <xsd:complexContent>
    <xsd:restriction base="base">
      <xsd:attributeGroup ref="attG"/>
    </xsd:restriction>
  </xsd:complexContent>
</xsd:complexType>

<xsd:attributeGroup name="attG">
 <xsd:attribute name="a" use="prohibited"/>
</xsd:attributeGroup>

Should the complex type "derived" still have attribute "a"? 

>From the spec, it seems that it would allow attribute "a", since the
attribute-uses of the attributeGroup "attG" does not contain "a".

If the above understanding is correct. Having a prohibited attribute in
an attributeGroup is useless and does nothing, and it could be argued
that such attributes should be disallowed from attribute groups.


Thanks,
Zafar