[m-rev.] for review: factor out common code in const_prop.m

[Zoltan Somogyi]

On Thu, 08 Dec 2022 20:27:16 +1100 (AEDT), "Zoltan Somogyi" <zoltan.somogyi at runbox.com> wrote:

> The new code in const_prop.m is the test case for the fix
> to string trie switches in my last diff. Since it requires that fix,
> I will commit it only after that fix is in an rotd.

The fix is in last night's rotd, so I would like to commit this.

> For review by anyone.

Does anyone intend to review this? It shouldn't take long.

Zoltan.