old_contents: 'use strict';
  
  /**
   * Internal dependencies
   */
  let NormalDistribution = require( './normal-distribution' );
  
  // See: http://mathworld.wolfram.com/NormalDifferenceDistribution.html
  function NormalDifferenceDistribution( proportionA, proportionB ) {
  	let mean = proportionB.mean - proportionA.mean,
  		sd = Math.sqrt( proportionA.variance + proportionB.variance );
  	NormalDistribution.call( this, mean, sd );
  }
  
  NormalDifferenceDistribution.prototype = Object.create( NormalDistribution.prototype );
  NormalDifferenceDistribution.prototype.constructor = NormalDifferenceDistribution;
  
  module.exports = NormalDifferenceDistribution;
  
new_contents: 'use strict';
  
  /**
   * Internal dependencies
   */
  let NormalDistribution = require( './normal-distribution' );
  
  // See: http://mathworld.wolfram.com/NormalDifferenceDistribution.html
  class NormalDifferenceDistribution extends NormalDistribution {
  	constructor( proportionA, proportionB ) {
  		let mean = proportionB.mean - proportionA.mean,
  			sd = Math.sqrt( proportionA.variance + proportionB.variance );
  		super( mean, sd );
  	}
  }
  
  module.exports = NormalDifferenceDistribution;
  
subject: Convert Normal Difference Distribution class to ES6