Looking for 18.1(b)

Not long ago during a presentation I was asked to give an example of rule 18.1(b). I must confess I was not able to come up with one that was not also already covered by 18.1(a). I've been thinking about it after that, but could use some help.

Lets start at the beginning:

Rule 18.1 is the rule that tells us when rule 18 applies. There are four bullets, from (a) to (d):
The first one is easy enough. For boats on different tack on a beat, approaching a windward mark, there's no rule 18, until one of them changes tack

The third one has been added in the latest rule cycle, but most people already applied it before. No rule 18 between a boat approaching and a boat leaving a mark.

The fourth bullet (d) tells us that in case the mark is also a continues obstruction, rule 19 applies and not rule 18.

That leaves the second one, rule 18.1(b):
Rule 18 does not apply between boats on opposite tacks when the proper course at the mark for one but not both of them is to tack.

The one I came up with was a starboard tack boat on a reach toward a mark and a port tack boat on a beat about to round the same mark. Something like this:

Rule 18 is off until the Orange boat passes head to wind. Then she becomes inside boat entitled to mark-room. If Purple has to avoid Orange before she's passed head to wind; penalty on Orange
If Purple only has to respond after Orange has passed head to wind, no infringement, no penalty.

READ UPDATE in Comments!
(J. 23/02/10; 21:33 h)

Can you find other situations where rule 18.1(b) is applicable?