Build a visual index of multiple PNM images. See also:
pamundice. More information: https://netpbm.sourceforge.net/doc/pnmindex.html.
pnmindex {{path/to/input1.pnm path/to/input2.pnm ...}} > {{path/to/output.pnm}}
pnmindex {{[-s|-size]}} {{50}} {{path/to/input1.pnm path/to/input2.pnm ...}} > {{path/to/output.pnm}}
pnmindex {{[-a|-across]}} {{10}} {{path/to/input1.pnm path/to/input2.pnm ...}} > {{path/to/output.pnm}}
pnmindex {{[-c|-colors]}} {{512}} {{path/to/input1.pnm path/to/input2.pnm ...}} > {{path/to/output.pnm}}