Rule 18.2 (a) and (b)

Rule 18.2( a ) is supposed to be the general case and rule 18.2( b ) is supposed to be an exception or special case.  It turns out in practice that rule 18.2( b ) situations are the most commonly occurring.  Maybe the rule could be re-written by putting 18.2 (b ) first.

An example of when rule 18.2 ( b ) never applies at all is when a boat clear ahead passes outside the zone and the boat clear astern reaches the zone first (Case 2).

An example of when 18.2 ( b ) applied initially and then ceased to apply and rule 18.2 ( a ) 'takes over' are where a boat entitled to mark-room passes head to wind, then passes back through head to wind onto her original tack and boats are again overlapped (rule 18.2( c ) last paragraph).