The subject invention pertains to systems and methods for iterating over or otherwise traversing a collection of objects or elements. More specifically, mechanisms are provided for handling both conventional non-nested streams such as enumeratables as well as nested streams. Nested streams provide a more efficient mechanism for traversing certain objects such as recursive generators. One problem with nested streams is that they need to be flattened or simplified. Techniques are provided herein to allow such streams to be flattened quickly and efficiently. Moreover, systems and methods are disclosed to preserve backward compatibility of optimized nested stream processing with conventional non-nested streams.