def tx_lines_by_period_cycle_init(mod, period, cycle):
    """
    Re-arrange the 3-dimensional PRDS_CYCLES_TX_DCOPF set into a 1-dimensional
    set of TX_DCOPF, indexed by PRD_CYCLES.
    """
    txs = list(tx for (p, c, tx) in mod.PRDS_CYCLES_TX_DCOPF
               if c == cycle and p == period)
    return txs