[plt-scheme] Highlighting parentheses.

Well, I couldn't resist (altho I probably should have). I've added
this feature to DrScheme, but have currently checked a version to SVN
that has it disabled.

Here are two screenshots, showing possibilities:

  http://people.cs.uchicago.edu/~robby/tmp/gray-parens.png
  http://people.cs.uchicago.edu/~robby/tmp/blue-parens.png

If you are interested and use SVN, please look at the bottom
of plt/collects/framework/private/color.ss, around like 754. There you
will see some code that builds a vector of  colors. DrScheme will use
the colors in the vector to highlight the different depths of nested
sexps, stopping with the last color when the depth is larger than the
number of colors in the vector. (Thus, when the vector only contains a
single color, you get the current behavior.)
Attached are two examples (those are the two commented out vectors in
the source code).

If people supply a few nice looking vectors, I'll turn this choice
into a preference for future versions of DrScheme.
Robby