create a pair of arbitrary array:
    a:=matrix(2,2,max) b:=a+3

    a=|0 1|   b=|3 4|
      |1 1|     |4 4|

concatentate them vertically:
                 |0 1|
    stack(a,b) = |1 1|
                 |3 4|
                 |4 4|

    augment(a,b) = |0 1 3 4|
                   |1 1 3 4|