old_contents: function each (arr, iterator, callback) {
    var idx = 0
    var len = arr.length
    var results = []
  
    function done (err, res) {
      var i = idx++
  
      if (err) return callback(err, results)
      if (res) results[i] = res
      if (idx < len) return iter.call(this)
  
      return callback(err, results)
    }
  
    function iter () {
      iterator(arr[idx], idx, done)
    }
  
    iter()
  }
  
  module.exports = each
  
new_contents: function each (arr, iterator, callback) {
    var i = 0
    var len = arr.length
    var results = []
  
    function done (err, res) {
      results[i] = res
      i++
  
      if (err) return callback(err, results)
      if (i === len) return callback(err, results)
  
      return iter.call(this)
    }
  
    function iter () {
      iterator(arr[i], i, done)
    }
  
    iter()
  }
  
  module.exports = each
  
subject: refactor series-each