<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def unpack(data):
    '''unpack from delimited data'''
    size, position = decoder._DecodeVarint(data, 0)
    envelope = wire.Envelope() 
    envelope.ParseFromString(data[position:position+size])
    return envelope