"I feel uneasy about the shallow change to +; I'm sure the same bug exists in some other functions since deep niltree is the default."
From what I can see, there are only three places in the pg-Arc code where 'ac-niltree traverses too far, and they're all in ac.scm. Two are the definitions of + and ar-+2, and one is a misleading comment:
; Arc primitives written in Scheme should look like:
; (xdef foo (lambda (lst)
; (ac-niltree (scheme-foo (ar-nil-terminate lst)))))
; That is, Arc lists are NIL-terminated. When calling a Scheme
; function that treats an argument as a list, call ar-nil-terminate
; to change NIL to '(). When returning any data created by Scheme
; to Arc, call ac-niltree to turn all '() into NIL.
; (hash-table-get doesn't use its argument as a list, so it doesn't
; need ar-nil-terminate).
From another point of view, there are only a few places where 'ac-niltree probably needs to be recursive. Those are in the definitions of 'ac-call, 'ac-mac-call, and 'ac-macex, where they deal with quotation and macroexpansion, the two ways literal code is made available to Arc programs.
The other uses of 'ac-niltree are in 'ar-coerce (string to cons), 'dir, and 'timedate, where the lists are flat anyway.
I only looked for uses in pg-Arc, not any code that's been derived from it.
