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

One more note:

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???

Maybe try?

a3 <- structure(1:10, dim = c(2,5), class = "array")

Then class(a3) gives "array", and is.matrix and is.array are both still 
TRUE.

Erik