[m-rev.] for post-commi review: unify erlang implementation of mutables

[Peter Wang]

On Fri, 30 Jan 2015 00:06:39 +1100 (EST), "Zoltan Somogyi" <zoltan.somogyi at runbox.com> wrote:
> For review by anyone. However, what is worth reviewing is not
> so much this diff, as the contents of add_mutable_aux_preds.m
> after its installation.
> 
> I also noticed that the code implementing mutables for the Erlang
> backend assumed that trailing was not enabled. I checked, and I found
> comments in the compiler that say that the Erlang backend does not
> implement trailing, but I can find nothing in handle_options.m to generate
> an error if users try to compile something with --grade erlang --use-trail.
> Is this intentional? Peter, do you know?

I'd never have thought to try that.  We also don't warn about lack of
trailing support for Java and C#.  Similarly for profiling, and perhaps
tabling.

Peter