#compdef rgb3toppm

local expl

if [[ CURRENT -eq 2 ]]; then
  expl='red picture file'
elif [[ CURRENT -eq 3 ]]; then
  expl='green picture file'
elif [[ CURRENT -eq 4 ]]; then
  expl='blue picture file'
fi

if [[ -n "$expl" ]]; then
  _description expl "$expl"
  _files "$expl" -g '*.(#i)pgm'
fi