code: function mixin(a) {
  	var k, key, o;
  	var len = arguments.length;
  
  	for (k = 1; k < len; k += 1) {
  		o = arguments[k];
  
  		for (key in o) {
  			var v = o[key];
  
  			if (o.hasOwnProperty(key)) {
  				if (v === undefined || v === null) {
  					delete a[key];
  				} else {
  					a[key] = o[key];
  				}
  			}
  		}
  	}
  
  	return a;
  }
docs: mixin(a, ...sources)