#!/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
  [ -z "$1" ] && merge_pair "$first" "$second" || \
    merge_pair "$first" "$second" | merge_set /dev/stdin "$@"; }
[ -n "$2" ] && merge_set "$@" || { [ -n "$1" ] && cat "$1"; }