[NTG-context] Extract only certain part of document (via modes)?

Dear list, and in particular Hans, I asked the same question on stackexchange before today (http://tex.stackexchange.com/q/337738/52406), and got the answer that what I want to do might not be possible. In my large document I have exercises (typeset with \startexercise \stopexercise, defined as an enumeration). Is it possible to use modes (or any other trick) to be able to compile the same file and get only the exercises. For example, context file.tex should give the full document, while context --mode=exercises file.tex should give only all the exercises (or just everything "in one mode")? /Mikael