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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 outpoint(tx_id, index, tree=None):
    '''
    hex_str, int, int -> Outpoint
    accepts block explorer txid string
    '''
    tx_id_le = bytes.fromhex(tx_id)[::-1]
    return tb.make_outpoint(tx_id_le, index, tree)