Trying a different approach.  Feels more like tree recursion, but I can't really
follow the flow yet.
Getting what looks to be a similar wrong answer to earlier this evening, but via a whole
different way :-)
I seem to have broken applyMove somewhere along the line.  I am still checking this in
because I think the main solver is moving in the right direction.
Fixed unit tests which were broken when types changed.
Left one fork of the accumulator not updated.  Fixed.