Extend function that plays nice with setters and getters

extend.js

function extend(obj) {                                                                    
  var slice = Array.prototype.slice;                                            
                                                                                
  slice.call(arguments, 1).forEach(function(source) {                           
    var getter                                                                  
      , setter;                                                                 
                                                                                
    for (var key in source) {                                                   
      getter = source.__lookupGetter__(key);                                    
      setter = source.__lookupSetter__(key);                                    
                                                                                
      if (getter || setter) {                                                             
        getter && obj.__defineGetter__(key, getter);   
        setter && obj.__defineSetter__(key, setter);                   
      }                                                                         
                                                                                                                                                      
      else {                                                                    
        obj[key] = source[key];                                                 
      }                                                                         
    }                                                                           
  });                                                                           
                                                                                
  return obj;                                                                      
}