Whereas stopchunk() will stop the parser just as it enters the declared
chunk, stoponexit() will stop just before it leaves the chunk. this is
useful for finding the end of forms, which would indicate that you've
collected all possible data in this FORM and may now act on it.

    /* Ask ParseIFF() to stop with IFFERR_EOC when leaving a FORM ILBM */
    StopOnExit(iff,ID_ILBM, ID_FORM);