The new --layout parameter would allow the following values:

• 2x2 : meaning to combine input files on every page of the output PDF document so that it forms a grid of 2 columns & 2 rows ( this is similar to https://github.com/pmaupin/pdfrw/blob/master/examples/4up.py )
• 3x3 : same, with a grid of 3 columns & 3 rows
• 1x2 : same, with a grid of 1 column & 2 rows
• 2x1 : same, with a grid of 2 columns & 1 row