cogen: python coroutine library introduction

Ok, introductory stuff: before python 2.5 generators were just a unidirectional computation structure. That means one could get values out of the generator. In python 2.5 we have the enhancements from PEP 342: Coroutines via Enhanced Generators – generators have 3 extra methods: sent, throw and close and the yield statement is a expression now. [...]