subscribeOrderBook (symbol, prec = 'P0', len = '25', freq = 'F0') {
    const filter = {}

    if (symbol) filter.symbol = symbol
    if (prec) filter.prec = prec
    if (len) filter.len = len
    if (freq) filter.freq = freq

    this.subscribe('book', symbol, filter)
  }
@param {string} symbol - symbol for order book
@param {string} [prec] - precision, i.e. 'R0', default 'P0'
@param {string} [len] - length, default '25'
@param {string} [freq] - default 'F0'