missing input cells in notebooks

[dandrake]

I'm looking at, for example, 02_pmf.ipynb and many input cells are empty when I don't expect them to be -- for example, below "To solve this problem, I'll use np.arange to represent 101 hypotheses, numbered from 0 to 100." I'd expect to see

np.arange(0, 101)

but instead it's blank:

I know some of the cells are blank because the reader is supposed to do something, but many of them seem to be part of the text where you intend to have some code.