Follow OCaml 'interface' rule
Follow OCaml 'interface' rule

See the interface guidelines on
http://caml.inria.fr/pub/docs/manual-ocaml-4.04/intfc.html
Paragraph 19.5.1 states the following:

Rule 1 A function that has parameters or local variables of type value must
begin with a call to one of the CAMLparam macros and return with CAMLreturn,
CAMLreturn0, or CAMLreturnT. In particular, CAMLlocal and CAMLxparam can only be
called after CAMLparam.

Although this is a callback function, add a call to CAMLparam(0)

paulp/unison