#!/bin/sh

merge_pair() {
  { tee /dev/stderr < "$1" | diff -n - "$2" | sed -n '3,$p'; } 2>&1
}

merge_set() {
  first="$1"
  second="$2"
  shift 2
  if [ -z "$1" ]; then
    merge_pair "$first" "$second"
  else
    merge_pair "$first" "$second" | merge_set /dev/stdin "$@"
  fi
}

if [ -n "$2" ]; then
  merge_set "$@"
else
  [ -n "$1" ] && cat "$1"
fi