help with dated entailments ... (for POWDER)

In my analysis of what is going in POWDER I have now written

http://www.w3.org/2007/OWL/wiki/POWDER

I have a question.

POWDER includes two properties wdr:validFrom and wdr:validUntil.

Inbetween these dates then an example like:


http://www.w3.org/2007/OWL/wiki/POWDER#Example

should 'entail'

http://www.w3.org/2007/OWL/wiki/POWDER#The_subclass_relationship

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Outside these dates the 'entailment' is not known to hold.

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Clearly a full formal treatment of time is well outside the scope of 
what will be included in the POWDER specification.

So how do I hack it?

Or perhaps more positively, how do I express pragmatically the 
relationships.

I have to define a semantic extension to RDF, so I have quite a lot of 
wiggle room, but need to use it cautiously.

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I'll try writing a straw man and adding it to the page.

Might take an hour or two.

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Note for the DL people - I need to also articulate how my approach 
relates to OWL DL.

Jeremy