rectangle

val dimens = Dimens.rectangle(length = 4, width = 3)
    0    1    2    3
0  [0]  [1]  [2]  [3]
1  [4]  [5]  [6]  [7]
2  [8]  [9]  [10] [11]

dimens.size = 12 // size of population
dimens.count = 2 // rectangle is two-dimensional space