human
Solve the following python programming problem given the following code:
# %load asyncio_call_at.py
import asyncio
import time

def callback(n, loop):
    print('callback {} invoked at {}'.format(n, loop.time()))

async def main(loop):
    now = loop.time()
    print('clock time: {}'.format(time.time()))
    print('loop  time: {}'.format(now))

    print('registering callbacks')
    loop.call_at(now + 0.2, callback, 1, loop)
    loop.call_at(now + 0.1, callback, 2, loop)
    loop.call_soon(callback, 3, loop)

    await asyncio.sleep(1)

event_loop = asyncio.get_event_loop()
try:
    print('entering event loop')
    event_loop.run_until_complete(main(event_loop))
finally:
    print('closing event loop')
    event_loop.close()

!python asyncio_call_at.py
Given the above code, generate some markdown instructions for it.
assistant
#Here is the Python program you requested.

In this example, the same callback function is scheduled for several different times with different arguments. The final instance, using call_soon(), results in the callback being invoked with the argument 3 before any of the time-scheduled instances, showing that “soon” usually implies a minimal delay.
Scheduling a Callback for a Specific Time
It is also possible to schedule a call to occur at a specific time. The loop uses a monotonic clock, rather than a wall-clock time, to ensure that the value of “now” never regresses. To choose a time for a scheduled callback it is necessary to start from the internal state of that clock using the loop’s time() method.