[r-t] calling grandsire round

[Mark Davies]

> any lead end could be reached in seven leads.

Seven leads gives you a bound of 3^7 + 3^6 + ... + 3 callings, which is
certainly ample. Should be possible to prove this one way or another
(without considering all cases) with a variant on the coursing order
argument - probably start with b-blocks and consider moving target bells
forward through the order.

MBD