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Rosser's definition of addition does not work for Parigot encoding, because
when we call (n X s) below, n will give its predecessors to s, not the 
predecessors of n + m.

paddb : pnat → pnat → pnat = λ n : pnat , λ m : pnat , [pnat] λ X : * , λ s : pnat → X → X , λ z : X , ([pnat!] n X s ([pnat!] m X s z)).

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/usr/bin/time --format='%e %M'

Note that %M values are fourt times too big, as described here:

https://groups.google.com/forum/#!topic/gnu.utils.help/u1MOsHL4bhg)

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Stump-Fu mergesort for lists seems too slow.  The culprit seems to be
converting lists to Braun trees.  Compare stumpfu/sorttest/test.rkt
and cbv-parigot/sorttest/test.rkt.
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