Nicer log2 function for nat (suggested by Hugo)
Nicer log2 function for nat (suggested by Hugo)

 The auxiliary variable q is now increased continuously instead
 of being doubled from time to time. Interest: this version is
 obviously linear, and specification proofs are slightly simplier.

 NB: the previous version was in fact also linear I think, but
 proving this requires a proper complexity analysis.

 I'm sure this algorithm is related with some cellular automata
 stuff in the spirit of the firing squad :-)

git-svn-id: 44dc50bdcc0f5d1a212c497508dc9452e0b35e2b@13720 85f007b7-540e-0410-9357-904b9bb8a0f7

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