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What is the reason for raising ValueError, instead of
returning True right away?

Please re-read section 14.25 of the RFC: a if-modified-since
date "from the future" must be considered invalid.

Also, there seems to be some interaction with the Range:
header. How is that dealt with?

Also, where is the length attribute to if-modified-since
specified?

Also, should there be changed to Doc/lib/libsimplehttp.tex?