in this article
http://code.google.com/intl/de-DE/appengine/articles/paging.html
the author points out the problems that arise when you use a field
that may not be unique for paging.
the solution is to use a sharded counter over the user to make the
field unique.

Very fine until here.
But then he suggests to use a MD5-hash-value of the unique value
instead of the real unique value.

This is obviously wrong:
A hash function, will by definition NOT retain the uniqueness of the
source value!

Sure, the chances that 2 unique values result in the same hash value
is (and should by definition be) very low:
but we are not satisfied with a "solution" that works most of the
time, are we?

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