code: function unifiedDiff (actual, expected, formatAdd, formatRem) {
    return [
      formatAdd('+ expected'),
      formatRem('- actual'),
      ''
    ]
      .concat(
        diff.createPatch('string', actual, expected)
          .split('\n')
          .slice(4)
          .filter(function (line) {
            return line[0] === '+' || line[0] === '-'
          })
          .map(function (line) {
            return line[0] === '+' ? formatAdd(line) : formatRem(line)
          })
      )
      .join('\n')
  }
docs: Return unified diff of actual vs expected.
  
  @param {*} actual Actual value
  @param {*} expected Expected value
  @param {function} formatAdd Addition formatter
  @param {function} formatRem Removal formatter
  @return {string}