If the record update { expr with f1 = ...; f2 = ... } updates
each field in the record, the intermediate variable for expr
is unused. Avoid the warning by naming this variable "_q ".

It should be noted that 2ff8efca disables the "forgotten diverges"
warning for Eabstr, which changes the behaviour, but is a good idea
anyway.

as follows:

maps:   permut_sub     -> permut
arrays: map_permut_sub -> permut     (to be consistent with maps)
        permut_sub     -> permut_sub
        permut         -> permut_all

Ca m'apprendra a commiter/pusher trop vite...

grants a long-standing feature wish by Andrei

this is a contribution of Laurent Théry

predicates permut over maps and arrays are given new semantics, as follows:
- MapPermut: permut m1 m2 l u means that m1[l..u[ is a permutation of
  m2[l..u[ and values outside the interval [l..u[ are *ignored*.
- ArrayPermut: permut_sub a1 a2 l u means that a1[l..u[ is a permutation
  of a2[l..u[ and other meaningful values are *identical*.
- ArrayPermut: another predicate map_permut_sub has the same semantics as
  MapPermut.permut_sub, that is values outside of the interval [l..u[
  are ignored