Section 3 says:
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Optionally, the data source MAY return an updated enumeration context in the Pull response; when present, this new enumeration context SHOULD replace the old one on the consumer, and SHOULD be passed to the data source in all future responses until and unless the data source again returns an updated enumeration context.

Consumers SHOULD NOT reuse old enumeration contexts that have been replaced by the data source. Using a replaced enumeration context in a Pull response MAY yield undefined results, including being ignored or generating a fault. 
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(Note: the word "responses" in the first para should be "requests")

And then in section 3.2 it says:
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[Body]/wsen:PullResponse/wsen:EnumerationContext

    The OPTIONAL EnumerationContext element, if present, contains a new XML representation of the current enumeration context. The consumer is required to replace the prior representation with the contents of this element. 
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Its not clear how strongly the spec mandates that the new enum context
be used in place of the old one.  The SHOULDs seem to contradict the "required"
in the PullResponse.

Proposal:
remove this ambiguity by mandating that the new Enum Context MUST be used
by changing the text in section 3 with:
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Optionally, the data source MAY return an updated enumeration context in the Pull response; when present, this new enumeration context MUST replace the old one on the consumer, and MUST be passed to the data source in all future requests until and unless the data source again returns an updated enumeration context.

Consumers MUST NOT reuse old enumeration contexts that have been replaced by the data source. Using a replaced enumeration context in a Pull response might
yield undefined results, including being ignored or generating a fault. 
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and replace the text in the PullResponse with:
The consumer MUST replace the prior representation with the contents of this element.