Optional Mapping Argument to matrix() Function?

[mark4flies]

The latest User Guide describes the matrix(r,c) function as having an optional third argument that is either a value or a mapping of (j,k). The example shows how to use it with a constant but not with a mapping. It is not in the catalog of functions so there is no further help or examples. How do define a mapping of (j,k) for this argument?
Thanks!

[Didier Lachieze]

Some examples:

PHP Code:

matrix(3,5,(j,k)->1+j+2*k)
matrix(5,5,(j,k)->ifte(j>k,0,rand(1000))) 


[mark4flies]

Thanks so much!