Enum has this text for the MaxTime element:
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The data source MUST recognize the wsen:MaxTime element and return a wsen:TimedOut fault if no elements are available prior to the request message's deadline. However, this fault MUST NOT cause the enumeration context to become invalid and the consumer can issue additional Enumerate requests using this enumeration context after receiving this fault. 
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This is problematic due to the Pull() and Enumerate() now being
part of the same operation.  If the MaxTime is hit is during the
Enumerate/Create step a fault is thrown - this means that the client
will never get back the EnumerationContext for the new enum (which is still 
active). They would have no choice but to create an entirely new Enum - which
may not be what they want - and it means we have zombie enums left around.

Proposal:
- Modify Enum so that when MaxTime is hit the data source just returns
  zero Items in the response instead of a fault.
- remove the TimedOut fault
- modify the xsd so that Items can be empty