the definition of what is officially a sparse vector to say that unsigned
integral keys are required.  Having this requirement is nice because it creates
a simple correspondence between dense vector index values and sparse vector keys.
The previous sparse vector definition was just excessively generic.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403523