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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def heappop(heap):
 """Pop the smallest item off the heap, maintaining the heap invariant."""

    lastelt = heap.pop()    # raises appropriate IndexError if heap is empty
    if heap:
        returnitem = heap[0]
        heap[0] = lastelt
        _siftup(heap, 0)
        return returnitem
    return lastelt