Essential things to remember from today's class (class 23, 9th November)

Be sure you understand how the recursion made handling sub-formulas almost
   trivial, why the input object had to be a reference parameter, and
   why we needed an input object in the first place.

Understand the setw(depth * 3) << "" trick and the extra depth parameter.

You should be able to reconstruct the fractions program for yourself,
nothing has got complicated there yet.