thing that annoys me is there is nothing for arc that is anything close to what slime is for cl. i would settle for an inferior arc mode. my emacs skills are not good enough to take this into my own hands, i'm afraid.
From what I've read, pg uses vi, so I expect he's developed a filetype for Arc. I was actually a heavy vim user for a couple years, but recently switched to Emacs in part because of the Lisp support. I was back to my vim productivity level in about 3 days, so maybe you could give it a shot :)
No clue... I'm totally unfamiliar with Xemacs. Likely it just has slightly different syntax for syntax-tables. I think that line was copied from lisp.el, though... maybe whatever Xemacs' equivalent file is has a compatible version.
The only other reference to "modify-syntax-entry" I found was in slime.el, both for Emacs and XEmacs. lisp.el doesn't have it. There were only two "modify-syntax-entry" lines used, vs. the many in arc.el. Lines 6451-6462 from slime.el:
(let ((table (copy-syntax-table lisp-mode-syntax-table)))
;; We give < and > parenthesis syntax, so that #< ... > is treated
;; as a balanced expression. This enables autodoc-mode to match
;; #<unreadable> actual arguments in the backtraces with formal
;; arguments of the function. (For Lisp mode, this is not
;; desirable, since we do not wish to get a mismatched paren
;; highlighted everytime we type < or >.)
(modify-syntax-entry ?< "(" table)
(modify-syntax-entry ?> ")" table)
"Syntax table for SLDB mode.")