Paper-folding is a recurring theme in mathematics. The art of origami is much-loved by many who also enjoy recreational maths. One particular folding problem is remarkably easy to state, but the solution remains elusive:

**Given a map with M ****×**** N panels, how many different ways can it be folded? **

Each panel is considered to be distinct, so two foldings are equivalent only when they have the same vertical sequence of the L = M *×* N layers.