[m-rev.] for review: Add maybe_default/2 standard library function

[Zoltan Somogyi]

On Mon,  1 Aug 2016 15:57:38 +1000, Paul Bone <paul at bone.id.au> wrote:
> +    % maybe_default(_, yes(X)) = X.
> +    % maybe_default(X, no) = X.

I would delete those lines. Otherwise, the diff is fine.

Zoltan.