[NTG-context] chapter-like headings with separate numbering

I'm experimenting with making a heading that is like a chapter in format but has its own numbering (or lettering). The purpose to have an interlude, or extended example, every few chapters. The following almost-minimal example almost does so. However, the second interlude is numbered with 'A' instead of 'B'. Is there a missing key=value pair, or is the approach completely wrong? \definesection[nc] \setupsection[nc][conversion=Characters] \definehead[interlude][section=nc,default=chapter] \setuphead[interlude][incrementnumber=yes] \starttext \title{Contents} \placelist[chapter,interlude] \chapter{Chap 1} \interlude{Interlude 1} \chapter{Chap 2} \interlude{Interlude 2} \stoptext