See discussion in github issues.
#19

I rewrote the logic here to avoid anything more
complicated that pushing or poping pages.

I struggled to find algorithm that was symetric
between old and new pages. Both loops now 
preserve the prefix (pages that are matching)
which I moved out of logic and into a variable.

This was expressed as an 'unless' statement but
coffeescript messed up the application of 
demorgan's law.

This was only used once, very early. I've hunted
for the spot but can't find it. The error message
should be clear to anyone who knows that we've
removed this global modification.

This is the first operation that depends on the
lineup being correct. It makes sense that it 
should be first. It struggled (and still does some)
to pull all the pieces together.