code: function factory(ctx, key) {
    return unescape
  
    // De-escape a string using the expression at `key` in `ctx`.
    function unescape(value) {
      var prev = 0
      var index = value.indexOf(backslash)
      var escape = ctx[key]
      var queue = []
      var character
  
      while (index !== -1) {
        queue.push(value.slice(prev, index))
        prev = index + 1
        character = value.charAt(prev)
  
        // If the following character is not a valid escape, add the slash.
        if (!character || escape.indexOf(character) === -1) {
          queue.push(backslash)
        }
  
        index = value.indexOf(backslash, prev + 1)
      }
  
      queue.push(value.slice(prev))
  
      return queue.join('')
    }
  }
docs: Factory to de-escape a value, based on a list at `key` in `ctx`.