ppmmix --version (return code: 0)
ppmmix: Using libpbm from Netpbm Version: Netpbm 10.0
ppmmix: BSD defined
ppmmix: RGB_ENV='RGBDEF'
ppmmix: RGBENV= 'RGBDEF' (env vbl is unset)
ppmmix -h (return code: 1)
usage: ppmmix fadefactor ppmfile1 ppmfile2
fadefactor: 0.0 = only ppmfile1, 1.0 = only ppmfile2