? in displayed equations in LE2 1.2.3

[tlofaro]

question marks instead of number show up in parts e-g of LE2 1.2.3.

[siwelwerd]

This is a "feature" of the book, or at least an intentional choice--we use the question marks in several places to build in some scaffolding for the students. They should be read as a fill-in-the-blank. Perhaps we can add a note to the instructor version explaining this so instructors aren't taken by surprise and can explain that to their students, or maybe an <aside> that shows up for the students as well?