[racket] auto-spacer

Cute!

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Of course, this is just recreating TeX's \halign.  Since the modern
TeX is Scribble, as soon as Racket adopts Scribble as its layout
language in the editor, this should be trivial to do. (-:

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<random>

Last summer my student Brendan Hickey worked with me to prototype a
Scheme editor in JavaScript using content-editable, where each
expression is a span.  This lets you write things like

<monospace>
  (f   (+ (* 2 3)   (- 1 2)   (g x
          (* 4 5))               y
                                 z))
</monospace>

with appropriate paren matching.  Every box has > and v arrows, so you
can decide whether you want "traditional" or "boxy" alignment.  The
nice thing about this is that our monitors are much wider than they
are tall, so we get to have wider and shorter programs.

</random>

Shriram