code: function SwitchRW(valrw, cases, opts) {
      if (!(this instanceof SwitchRW)) {
          return new SwitchRW(valrw, cases, opts);
      }
      opts = opts || {};
      this.valrw = valrw;
      this.cases = cases;
      this.cons = opts.cons || Pair;
      this.valKey = opts.valKey || '0';
      this.dataKey = opts.dataKey || '1';
      // istanbul ignore if TODO
      if (opts.structMode) {
          this.poolReadFrom = this.structReadFrom;
      }
  
      BufferRW.call(this);
  }
docs: TODO: cases should be an es6 map