fin n is a convenient way to represent \
1 .. n\
fin n can be seen as a n-uplet of unit.
F1 is the first element of
the n-uplet. If
f is the k-th element of the (n-1)-uplet,
FS f is the
(k+1)-th element of the n-uplet.
Author: Pierre Boutillier
Institution: PPS, INRIA 12/2010-01/2012-07/2012