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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 num2bytes(value, size):
 """Convert an unsigned integer to MSB-first bytes with specified size."""

    res = []
    for _ in range(size):
        res.append(value & 0xFF)
        value = value >> 8
    assert value == 0
    return bytes(bytearray(list(reversed(res))))