This fixes (-2) mod 2 being evaluated as nonzero.

All files are extracted to the target repository, even those
corresponding to files from the Why3 standard library.
Some of these files may be empty (e.g. ref__Ref.ml) when everything
is overridden by the OCaml driver. You can ignore these files when
compiling the extracted OCaml code.

everything is contained in the driver (ocaml.drv)
with possible references to OCaml modules contained in why3extract.cma
(currently why3__Prelude, why3__BigInt, why3__Map)