pnmarith --version (return code: 0)
pnmarith: Using libpbm from Netpbm Version: Netpbm 10.0
pnmarith: BSD defined
pnmarith: RGB_ENV='RGBDEF'
pnmarith: RGBENV= 'RGBDEF' (env vbl is unset)
pnmarith -h (return code: 1)
usage: pnmarith -add|-subtract|-multiply|-difference|-minimum|-maximum pnmfile1 pnmfile2