Right now Eventing (and Enum) define policy with:
  <wse:MaxExpires ...> xs:duration </wse:MaxExpires> ? 

This is fine for defining the max value but it seems like we should also
allow people to say what the minimum value is.  For example, what if someone
only support subscriptions for exactly one year.  Using the above they can
only say that the max is one year but the client will never know that they 
can't specify something less than a year either.

This becomes even more important with an endpoint that only support
indefinite expirations - meaning just PT0s (explicitly or implicitly).
Putting PT0S in the above doesn't tell the client that any value other than
PT0S will be rejected.

Proposal:

Change it to:

  <wse:ExpiresRange min="xs:duration" max="xs:duration" ... />

And mandate that min <= max.

Do this for both enum and eventing.