clean up pick-an-index for inclusion in a paper
fix type comment

(thanks, Sam!)
adjust plot so that it uses "rvm" for the verification model
scribble/base: add `#:{column,row,cell}-properties` arguments to `tabular`

The new arguments greatly simplify adding properties such as alignment
and (HTML) background colors to table cells. For example, to make a
table with 'right alignment for the left-hand column, 'center alignment
for the second column, and 'left alignment for all remaining columns:

 (tabular cells
          #:sep (hspace 1)
          #:column-properties '(right center left))

Also, make `color-property` and `background-color-property` recognized
as table-cell style properties.

Finally, implement horizontal alignment for text rendering.
Fix tc-let for mutable variables.

Closes PR 14415.
Remove unused clause arguments in tc-let-unit.
Make let correctly implement the TC-Let rule from the paper.
FIXUP add comments to functions