The INTERLISP read program treats square brackets as 'super-parentheses': a
right square bracket automatically supplies enough right parentheses to match
back to the last left square bracket (in the expression being read), or if none
has appeared, to match the first left parentheses,
e.g., (A (B (C]=(A (B (C))),
(A [B (C (D] E)=(A (B (C (D))) E).
Here's a document which goes over a variety of different notations (although the fact they say "there is no opening super-parenthesis in Lisp" seems to be inaccurate considering the above):
They favor this approach, which is also the one that best matches the way I intend for Parendown to work:
"Krauwer and des Tombe (1981) proposed _condensed labelled bracketing_ that can be defined as follows. Special brackets (here we use angle brackets) mark those initial and final branches that allow an omission of a bracket on one side in their realized markup. The omission is possible on the side where a normal bracket (square bracket) indicates, as a side-effect, the boundary of the phrase covered by the branch. For example, bracketing "[[A B] [C [D]]]" can be replaced with "[A B〉 〈C 〈D]" using this approach."
That approach includes what I would call a weak closing paren, 〉, but I've consciously left this out of Parendown. It isn't nearly as useful in a Lispy language (where lists usually begin with operator symbols, not lists), and the easiest way to add it in a left-to-right reader macro system like Racket's would be to replace the existing open paren syntax to anticipate and process these weak closing parens, rather than non-invasively extending Racket's syntax with one more macro.