Clojure's spelling and Clojure's semantics. Repeated — #_#_ a b c — discards
that many following forms, and that falls out of the recursion rather than
being counted: the discard reads *a form*, and the form it reads may itself
begin with a discard, so the outer one throws away what the inner one already
stepped past.
It belongs to read_form rather than to the sequence readers, which is what makes
it work in every position a form can appear — top level, inside a list or a
vector or a map, before or after a quote. The two loops that look for a closer
or for end of input skip it as well, because a discard is not an element and a
file ending in one has read everything there is to read.
A trailing #_ with nothing after it is an error, and it is the same error an
unterminated form already gives.
`(a b) came back as the unknown name "`" — precisely the failure the
reader's own header warns about for the apostrophe, one sigil over and
still open. Same fix: the sigil reads as a wrapper and the reader stays
dumb about what it means.
Clojure's ` ~ ~@ rather than Common Lisp's ` , ,@ because a comma is
already whitespace here, and every binding vector in the corpus is
written assuming that. Changing is_delim to free up the comma would
rewrite more of the language than macros are worth.
Backtick and tilde join is_delimiter so that a~b is two things and can
never be one name. No symbol in the corpus contains either character, so
closing the class costs nothing now and would cost a migration later.
