How to use the vega-util.accessorFields function in vega-util

function aggregateParams(_, pulse) {
  var key    = _.field,
      value  = _.value,
      op     = (_.op === 'count' ? '__count__' : _.op) || 'sum',
      fields = accessorFields(key).concat(accessorFields(value)),
      keys   = pivotKeys(key, _.limit || 0, pulse);

  // if data stream content changes, pivot fields may change
  // flag parameter modification to ensure re-initialization
  if (pulse.changed()) _.set('__pivot__', null, null, true);

  return {
    key:      _.key,
    groupby:  _.groupby,
    ops:      keys.map(function() { return op; }),
    fields:   keys.map(function(k) { return get(k, key, value, fields); }),
    as:       keys.map(function(k) { return k + ''; }),
    modified: _.modified.bind(_)
  };
}

prototype.transform = function(_, pulse) {
  var features = this._features,
      points = this._points,
      fields = _.fields,
      lon = fields && fields[0],
      lat = fields && fields[1],
      geojson = _.geojson || (!fields && identity),
      flag = pulse.ADD,
      mod;

  mod = _.modified()
    || pulse.changed(pulse.REM)
    || pulse.modified(accessorFields(geojson))
    || (lon && (pulse.modified(accessorFields(lon))))
    || (lat && (pulse.modified(accessorFields(lat))));

  if (!this.value || mod) {
    flag = pulse.SOURCE;
    this._features = (features = []);
    this._points = (points = []);
  }

  if (geojson) {
    pulse.visit(flag, function(t) {
      features.push(geojson(t));
    });
  }

  if (lon && lat) {
    pulse.visit(flag, function(t) {

function inputVisit(get) {
    var fields = array(accessorFields(get)),
        i = 0, n = fields.length, f;
    for (; i

function dependency(f) {
  if (!isFunction(f)) return false;
  const set = toSet(accessorFields(f));
  return set.$x || set.$y || set.$value || set.$max;
}

function update(_) {
  var expr = _.expr;
  return this.value && !_.modified('expr')
    ? this.value
    : accessor(
        datum => expr(datum, _),
        accessorFields(expr),
        accessorName(expr)
      );
}

function modified(f, pulse) {
  var k, v;
  for (k in f) {
    if (isFunction(v = f[k]) && pulse.modified(accessorFields(v)))
      return 1;
  }
  return 0;
}

function visitInputs(f) {
    array(accessorFields(f)).forEach(_ => inputs[_] = 1);
  }
  visitInputs(_.sort);