void() { echo "VOID: $1"; exit 2; }
trial() {
  local arm="$1" n="$2"
  local id="$arm-$n"
  case "$id" in
    a-1|a-2|a-3|b-1|b-2|b-3) ;;
    *) void "trial id [$id] invalid (arm=[$arm] n=[$n])" ;;
  esac
  echo "OK id=[$id] outf=[R/$id.out]"
}
for n in 1 2 3; do trial a "$n"; done
for n in 1 2 3; do trial b "$n"; done