newbie xml xsd modelling problem

[Tis Veugen]

Hi,
I have a modelling problem that I would like to make clear with these
xml-file and xsd-file.

The xsd-file:

<xsd:simpleType name="st1">
<xsd:restriction base="xsd:NMTOKEN">
<xsd:enumeration value="v1"/>
<xsd:enumeration value="v2"/>
<xsd:enumeration value="v3"/>
</xsd:restriction>
</xsd:simpleType>

<xsd:complexType name="ct1">
<xsd:choice>
<xsd:sequence>
<xsd:attribute name="type" type="st1" fixed="v1"/>
<xsd:element name="str" type="string"/>
</xsd:sequence>
<xsd:sequence>
<xsd:element name="str" type="string"/>
</xsd:sequence>
<xsd:sequence>
<xsd:attribute name="type" type="st1"/>
</xsd:sequence>
</xsd:choice>
</xsd:complexType>

<xsd:element name="my_ct" type="ct1"/>

The xml-file:

<my_ct type="v1">
<str> my other data </str>
</my_ct>

<my_ct>
<str> my data </str>
</my_ct>

<my_ct type="v2">
</my_ct>

<my_ct type="v3">
</my_ct>

PROBLEM:
How to prevent that the following lines would be allowed, according to the
3rd choice :
<my_ct type="v1">
</my_ct>
So, how should the xsd file look like?

Thanx,
Tis Veugen