![](https://secure.gravatar.com/avatar/b79687264c3e06cba13b54d05b12a9f5.jpg?s=120&d=mm&r=g)
15 Sep
2015
15 Sep
'15
5:23 a.m.
Hi, when you pass a undefined argument to \lfence (or \rfence and \mfence) you get the "unknown" alternative \setvalue{\??mathleft \s!unknown}{\setfalse\c_math_fenced_done\ifconditional\c_math_fenced_unknown\normalleft \nexttoken\fi} which doesn’t work here because \nexttoken is never set by \lfence but only by \left. Wolfgang