docs: function getVisitorsBySet(sets) {
    var visitorsToInclude = sets.reduce(function(visitors, set) {
      if (!transformSets.hasOwnProperty(set)) {
        throw new Error('Unknown visitor set: ' + set);
      }
      transformSets[set].forEach(function(visitor) {
        visitors[visitor] = true;
      });
      return visitors;
    }, {});
  
    var visitorList = [];
    for (var i = 0; i < transformRunOrder.length; i++) {
      if (visitorsToInclude.hasOwnProperty(transformRunOrder[i])) {
        visitorList = visitorList.concat(transformVisitors[transformRunOrder[i]]);
      }
    }
  
    return visitorList;
  }