[m-rev.] for review: Define behaviour of string.set_char etc on ill-formed sequences.

[Peter Wang]

On Mon, 11 Nov 2019 17:32:31 +1100, Mark Brown <mark at mercurylang.org> wrote:
> Hi Peter,
> 
> tests/hard_coded/string_set_char.m could perhaps be updated?

I've added a test case.

Peter