Arc doesn't really have arrays, so it doesn't have 2-dimensional arrays. One approach is to use a list of lists.
arc> (= plane '((a b c) (d e f) (g h i)))
((a b c) (d e f) (g h i))
arc> ((plane 0) 0)
a
arc> ((plane 0) 1)
b
arc> ((plane 0) 2)
c
arc> ((plane 1) 2)
f
But indexing linked lists is inefficient, so you could similarly nest hash tables.
Granted, neither solution is pretty. I suspect they aren't there because (as with many things) pg hasn't needed them yet himself.
