On 17 Feb 2016, at 13:34, Hans Hagen <pragma@wxs.nl> wrote:

That was a tricky one to nail down ... it related to the fact that have \{ and \} have a different meaning when we flush metapost text (makes me wonder if that is still needed)

this helps

\installmathfencepair
 \letteropenbrace \Lbrace
 \letterclosebrace \Rbrace