When printing, atoms in BACKQ-UNPARSE are passed to
BACKQ-UNPARSE-EXPR, which wraps them in a comma. This means that the
read/write invariance of, say, ``(1 ,x) and ``(let () ,x) isn't
preserved, since we get `(,1 ,x) and `(LET (SB-IMPL::BACKQ-COMMA ())
,X), respectively. Lovely.
I have a patch which seems to fix this problem, but I'd like to test
it a little more before sending it along. Still, it seems to show
that this case is orthogonal to "a" and "b".