marshall(stamp) {
      const argval = this._argval || NO_PARAMS,
        argops = this._argops;
      let item, i, op, mod;
      if (argops) {
        const n = argops.length;
        for (i = 0; i < n; ++i) {
          item = argops[i];
          op = item.op;
          mod = op.modified() && op.stamp === stamp;
          argval.set(item.name, item.index, op.value, mod);
        }
        if (argops.initonly) {
          for (i = 0; i < n; ++i) {
            item = argops[i];
            item.op.targets().remove(this);
          }
          this._argops = null;
          this._update = null;
        }
      }
      return argval;
    }
Internal method for marshalling parameter values.
Visits each operator dependency to pull the latest value.
@return {Parameters} A Parameters object to pass to the update function.