I have an element named "vendingMachine".  This may appear in one of two
places

<customerSite>
 <vendingMachines>

or

<plannedVisit>
 <installationTask>

Because of this I made an element named vendingMachine and simply referred
to it when needed <xs:element ref="vendingMachine"/>.  Various other parts
of my XML need to refer to a vending machine by its serial number.  My
questions are

1) How do I define a key on vendingMachine @serialNumber so that it is
unique within the whole document no matter where it appears
2) How do I define a reference to a vending machine which could be in either
place?

Within <xs:schema> I have the following
 <xs:key name="vendingMachineSerialNumberKey">
  <xs:selector xpath="vendingMachine"/>
  <xs:field xpath="@serialNumber"/>
 </xs:key>

and using the key:
 <xs:keyref name="visitActionVendingMachineSerialNumber"
refer="vendingMachineSerialNumberKey">
  <xs:selector
xpath="roundData/visitDay/plannedVisits/plannedVisit/tasks/*"/>
  <xs:field xpath="@vendingMachineSerialNumber"/>
 </xs:keyref>

but when I validate my XML in XmlSpy I get the following error:
Identity constraint visitActionVendingMachineSerialNumber did not match
within the scope of element <data>.  The referenced identity constraint
"vendingMachineSerialNumberKey" is out of scope

Can anyone help?

Thanks!

Pete