Extract an operation from transaction data.
    Return the extracted fields as a dict.

def op_extract(op_name, data, senders, inputs, outputs, block_id, vtxindex, txid):
    """
    Extract an operation from transaction data.
    Return the extracted fields as a dict.
    """
    global EXTRACT_METHODS

    if op_name not in EXTRACT_METHODS.keys():
        raise Exception("No such operation '%s'" % op_name)

    method = EXTRACT_METHODS[op_name]
    op_data = method( data, senders, inputs, outputs, block_id, vtxindex, txid )
    return op_data