Add & update notes on new grammar.

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+# Simplifying S-Expression Grammar
+
+Supporting dots as part of identifiers and number literals, especially
+at the beginning like in `...` or `.5`, but then special-casing a lone
+dot for improper list syntax, is a bit annoying.
+
+It makes my otherwise crystal clear and simple BNF grammar, as well as
+my parser implementation, a bit less simple, which is bothersome.
+
+Since I've already broken off heavily from traditional s-expressions,
+I've decided to retire the dot as the improper list marker.  Instead
+I'll be using the ampersand.
+
+```
+;; The good old
+(foo bar . baz)
+
+;; The new kid
+(foo bar & baz)
+```
+
+This is way more annoying to type, and I've not yet gotten used to the
+way it looks.  But you don't type improper lists frequently in code,
+and I think the looks are just a matter of familiarity.
+
+By the way, the ampersand is now illegal in identifiers.  It's totally
+reserved for this singular purpose in the grammar.
+
+The only common use I know of is exception types in R6RS, which I've
+never liked anyway.  Using the question or exclamation marks would be
+more intuitive, to express a questionable situation or a surprise!
+
+I might walk back on this eventually, but so far it seems OK.