BPMN21 — Adding child attributes and child elements in XSLT causes errors

The examples of Business Process Model and Notation (BPMN) Version 2.0 - Beta 2, (May 2010) are valid with the provided XML Schema: spec/BPMN/2.0/20100501/BPMN20.xsd. However, the spec/BPMN/2.0/20100501/BPMN20-ToXMI.xslt does not transform these models into a complete XMI file. For example UserTasks with an implementation and ioSpecification is defined in BPMN as follows:
<semantic:userTask implementation="##unspecified" completionQuantity="1" isForCompensation="false" startQuantity="1" name="Review Issue List" id="_6-66">
<semantic:incoming>_6-167</semantic:incoming>
<semantic:outgoing>_6-169</semantic:outgoing>
<semantic:ioSpecification>
<semantic:dataInput id="Din1276277381462"/>
<semantic:inputSet>
<semantic:dataInputRefs>Din1276277381462</semantic:dataInputRefs>
</semantic:inputSet>
<semantic:outputSet/>
</semantic:ioSpecification>
<semantic:dataInputAssociation id="_6-151">
<semantic:sourceRef>_6-139</semantic:sourceRef>
<semantic:targetRef>Din1276277381462</semantic:targetRef>
</semantic:dataInputAssociation>
</semantic:userTask>

After the transformation with "BPMN20-ToXMI.xslt" I get the following result (without the specified implementation):
<flowElements xmi:type="bpmnxmi:UserTask" id="_6-66" name="Review Issue List" outgoing="_6-169 " incoming="_6-167 " isForCompensation="false" startQuantity="1" completionQuantity="1">
<ioSpecification xmi:type="bpmnxmi:InputOutputSpecification">
<inputSets xmi:type="bpmnxmi:InputSet" dataInputRefs="Din1276277381462 "/>
<outputSets xmi:type="bpmnxmi:OutputSet"/>
<dataInputs xmi:type="bpmnxmi:DataInput" id="Din1276277381462"/>
</ioSpecification>
<dataInputAssociations xmi:type="bpmnxmi:DataInputAssociation" id="_6-151" targetRef="Din1276277381462 " sourceRef="_6-139 "/>
</flowElements>

I took the "Email Voting 2.bpmn" example from the spec/BPMN/2.0/examples/ZIP. The transformation of "Email Voting 2.bpmn" produces the following error messages:
/XSLT-Transformation/BPMN20-ToXMI.xslt; Line #8; Column #116; Cannot add attribute dataObjectRef after child nodes or before an element is produced. Attribute will be ignored.
OR
/XSLT-Transformation/BPMN20-ToXMI.xslt; Line #8; Column #116; Cannot add attribute implementation after child nodes or before an element is produced. Attribute will be ignored.
(similar messages for cancelActivity, dataObjectRef, messageRef, attachedToRef, isCollection)