Access element at [row, column].
Both row and column must be from 0 to 3 inclusive. A matrix is 4x4 array
of numbers, and you can access the individual elements using this function.
Note the standard math notation - row is the first index.
Access element at sequential index (0..15 inclusive).
A matrix is 4x4 array of numbers (16 numbers in total). You can access
the individual elements using "flattened" index with this.
The index is row+column*4.