The primary motivation for these improvements was avoiding a while/pop pattern for looping. A for-loop is a big improvement over copying a list, then repeatedly popping the 0th element. A lesser improvement is the use of ``a.intersection(b)`` instead of ``c = set(b); len(c) > len(c - a)``. The check for intersection is more clear, and it avoids an unnecessary set construction. https://github.com/python/cpython/pull/7999/