How to make a 2-dim array being "interpreted" as an array rather than as a matrix

Dear List

 A 2-dimensional array seems to be "interpreted" as a matrix whereas higher dimensional arrays are interpreted as arrays, e.g.

 > a1 <- array(1:8,c(2,2,2))
 > class(a1)

 [1] "array"

 > a2 <- array(1:4,c(2,2))
 > class(a2)

 [1] "matrix"

 If I write a generic function (apologies if this is the wrong word) on arrays as

 foo <- function(x) UseMethod("foo")
 foo.array <- function(x) sum(x)

 then this fails on a 2-dimensional array:

 > foo(a1)

 [1] 36

 > foo(a2)

 Error in UseMethod("foo") : no applicable method for "foo"

 Is there any way in which I can "force" a2 to being "interpreted" as having class "array" rather than "matrix" so that foo will work on a2???

 Thanks in advance
 S?ren