* explanation for example with lists of lists made confusing use of
  the word "contains" to mean "is built out of".

* wrong formula for slices with step.  Is it ok to use LaTeX formulas
  (which become images in the html document)?  This version needs one
  because it's based on a fraction.  Just writing "\code{(j-i)/k}" here would
  be ambiguous because it looks like a rounding-down-to-the-previous-integer
  division, which is not what we need here.  Of course we could write
  "\code{float(j-i)/k}" but it just looks confusing.