An element with more than one possible type

I am trying to code a XML Schema (XSD file) for a web service in such a way:

<items>
 <item name = "pen" color ="22">
 <item name = "pencil" smoothness = "10" >
</items>

It is to say, if name is "pen" , then use the "color" attribute.
When name is "pencil" then use "smoothness" attribute.

What I have done is to define a simple type for name attribute

<xs:simpleType name="nameType">
      <xs:restriction base="xs:string">
              <xs:enumeration value="pen"/>
              <xs:enumeration value="pencil"/>
      </xs:restriction>
</xs:simpleType>

Then I defined an abstract complex type for "item" element:

<xs:complexType name="itemType" abstract = "true">
      <xs:attribute name = "name" type = "nameType"/>
</xs:complexType>

I restricted the "name" attribute to "pencil":

<xs:complexType name="itemTypePencil">
      <xs:complexContent>
              <xs:restriction base="itemType">
                      <xs:attribute name = "name" type = "nameType"
fixed="pencil"/>
              </xs:restriction>
      </xs:complexContent>
</xs:complexType>

.... and finally I added an attribute "smoothness":

<xs:complexType name="itemTypePencilSmoothness">
      <xs:complexContent>
              <xs:extension base="itemTypePencil">
                      <xs:attribute name = "smoothness" type = "xs:int"/>
              </xs:extension>
      </xs:complexContent>
</xs:complexType>

For the "pen" is the same as "pencil". First I restricted the "name"
attribute to "pen":

<xs:complexType name="itemTypePen">
      <xs:complexContent>
              <xs:restriction base="itemType">
                      <xs:attribute name = "name" type = "nameType"
fixed="pen"/>
              </xs:restriction>
      </xs:complexContent>
</xs:complexType>

.... and finally I added the "color" attribute

<xs:complexType name="itemTypePenColor">
      <xs:complexContent>
              <xs:extension base="itemTypePen">
                      <xs:attribute name = "color" type = "xs:int"/>
              </xs:extension>
      </xs:complexContent>
</xs:complexType>

The problem is to define the type for "item" element. If I use
"itemType" as type it the XML does not validated because
"itemType" is abstract. I I use:

<xs:element  name="items">
      <xs:complexType>
              <xs:sequence>
                      <xs:choice>
                              <xs:element name="item" type="itemTypePenColor"/>
                              <xs:element name="item"
type="itemTypePencilSmoothness"/>
                      </xs:choice>
              </xs:sequence>
      </xs:complexType>
</xs:element>

the XML does not validate because the validator assumes that the
"item" type is the first defined (itemTypePenColor). If I
change the declaration of "item"  element then the validator
assumes that the type of "item" is "itemTypePencilSmoothness".

Do you know how to solve this problem?

Thanks in advance.

Pau