code: function mergeFormatCharacters(formatCharacters) {
    var merged = copy(DEFAULT_FORMAT_CHARACTERS)
    if (formatCharacters) {
      var chars = Object.keys(formatCharacters)
      for (var i = 0, l = chars.length; i < l; i++) {
        var char = chars[i]
        if (formatCharacters[char] == null) {
          delete merged[char]
        }
        else {
          merged[char] = formatCharacters[char]
        }
      }
    }
    return merged
  }
docs: Merge an object defining format characters into the defaults.
  Passing null/undefined for en existing format character removes it.
  Passing a definition for an existing format character overrides it.
  @param {?Object} formatCharacters.