[dev branch] amscd problems

[pkra]

E.g., \begin{equation*} \begin{CD} A @>f>> B @>g>> C @>h>> A[1]\\@| @A sAA @A t AA @|\\A @>f' >> B' @>g'>> C @>h'>>A[1] \end{CD} \end{equation*} used to work fine (e.g. v4-beta.7) but crashes on a recent dev branch build.

[dpvc]

I've made the PR for this.