Normalize top comments to start with ";;".

Leave a single ";" for commented expressions and output indications.
Remove spaces at end-of-lines.
Indentation fixes.
More comment fixes.
Improve macro definition.

It's more robust to use `let' to have the macro result in its own new
scope.  (Unrelated to the hygiene point, of course.)
C-escapes example.

Also, remove old-style "~n"s.
Use idiomatic quoting for identifiers.
Improve #; example.

Also, I removed the following expression since this is the beginning
when interaction is not shown.  Instead, I added a comment to explain
what #;s are useful for.
Clarify functional data structures.

Note that `set-add' is functional, clarify immutable hashes, add mutable
hash examples to contrast.
Width.

Make the whole text fit in 70 characters.
Better link for more reading.
`swap' -> `swap!'
Add boxes example.
Improve mutable/immutable comment, add `make-vector' example.
Improve list section.
Improve functions intro.

Add `λ', sugared/unsugared examples, optionals, keywords.
Improve `with-handlers' an exceptions description.
`require' gets all bindings, not just functions.

Also, explain the ' use.
Improve class section.
Much more on iteration in all forms.