As per my earlier question TeX capacity exceeded (save size), I am generating a fairly large index of .txt, .tex, and .pdf files. This process has worked for a long time, and the size of the index is ...
See MWE below. The symbol ! in the \index causes an error. This is only the case in align environments (not in equation). So it seems to have to do something with amsmath. If ! is put in an \mbox ...