-/** Descend down one level in terms of progress reporting; e.g. if
- * there is a task which is split up into N subtasks, each of which
- * report their progress from 0 to 100%, call descend() before executing
- * each subtask, and ascend() afterwards to ensure that overall progress
- * is reported correctly.
- *
- * @param a Fraction (from 0 to 1) of the current task to allocate to the subtask.
- */