/// Step (increment by one) counter named by arg, and
/// zeroes slave counter(s) for which it is the master.
/// NOTE sub-slaves not zeroed! That happens at slave's
/// first step 0->1. Seems to be sufficient.
/// Step (increment by one) counter named by arg, and
/// zeroes slave counter(s) for which it is the master.
/// NOTE sub-slaves not zeroed! That happens at slave's
/// first step 0->1. Seems to be sufficient.