docs: function extraProperties(obj, allowed)
  {
  	mod_assert.ok(typeof (obj) === 'object' && obj !== null,
  	    'obj argument must be a non-null object');
  	mod_assert.ok(Array.isArray(allowed),
  	    'allowed argument must be an array of strings');
  	for (var i = 0; i < allowed.length; i++) {
  		mod_assert.ok(typeof (allowed[i]) === 'string',
  		    'allowed argument must be an array of strings');
  	}
  
  	return (Object.keys(obj).filter(function (key) {
  		return (allowed.indexOf(key) === -1);
  	}));
  }