: this module defines a family of relations Zwf on type
 Z by
 (Zwf   c) = l x,y. c£ x  c £ y  x < y
 and establishes that this relation is well-founded for all c
 (lemma Zwf_well_founded). This lemma is automatically
 used by the tactic Correctness when necessary.
 When no relation is given for the variant of a loop or a recursive
 function, then (Zwf 0) is used i.e. the usual order
 relation on positive integers.