d5194f23fa
The yield-from that existed previously wasn't actually implementing the full complexity of "yield from": http://legacy.python.org/dev/peps/pep-0380/#formal-semantics ... this includes passing along errors, and many other things. Also removes the yield-from backport macro, since it does not seem possible at present to conditionally build macros. Thus, there is no longer yield-from on pre-python-3.3 systems. Includes updated docs and tests to reflect all this.