(Solution proposed by Laurent Théry, see also Section 13.4.1 of the Coq'Art)
Errata
Page 220, section 8.2.7, read :
"binary_word (n+p)" and "binary_word (p+n)"
Page 227, last paragraph. Erase the proposition "and to the fact ... auto"
Pagge 229, last paragraph. Read "(page 134)" instead of "(Sect. 134)"