class ConcurrentRunner:
    def __init__(self, max_concurrency=5):
        self.max_concurrency = 5
        self.tasks = []

    async def put(self, coro):
        """Starts a coroutine if there are 4 or less already running ones.
        """
        # blocks until there's room
        while len(self.tasks) >= self.max_concurrency:
            await asyncio.sleep(0)
        fut = asyncio.create_task(coro())
        self.tasks.append(fut)
        fut.add_done_callback(self.tasks.remove)
        return fut

    async def wait(self):
        """Wait for remaining routines to finish
        """
        await asyncio.gather(*self.tasks)