Patch Set 4: Looks good to me, but someone else must approve

(2 inline comments)

This is a fairly minor tidy-up over Patch Set 3 - the nextVisit() method sits better on the DeltaVisit class if you ask me.

The code looks good to me, tho I just wanted to say that personally I prefer lobbing around immutable strongly-typed objects - I find the code easier to follow. I can see that mutability is sometimes necessary  but I wouldn't like to think it was being used prematurely... my 2 cents :-)