old_contents: import {changeset, isChangeSet} from 'vega-dataflow';
  import {error, hasOwnProperty, truthy} from 'vega-util';
  
  export function dataref(view, name) {
    var data = view._runtime.data;
    if (!hasOwnProperty(data, name)) {
      error('Unrecognized data set: ' + name);
    }
    return data[name];
  }
  
  export function data(name, values) {
    return arguments.length < 1
      ? dataref(this, name).values.value
      : change.call(this, name, changeset().remove(truthy).insert(values));
  }
  
  export function change(name, changes) {
    if (!isChangeSet(changes)) {
      error('Second argument to changes must be a changeset.');
    }
    var dataset = dataref(this, name);
    dataset.modified = true;
    return this.pulse(dataset.input, changes);
  }
  
  export function insert(name, _) {
    return change.call(this, name, changeset().insert(_));
  }
  
  export function remove(name, _) {
    return change.call(this, name, changeset().remove(_));
  }
  
new_contents: import {changeset, isChangeSet} from 'vega-dataflow';
  import {error, hasOwnProperty, truthy} from 'vega-util';
  
  export function dataref(view, name) {
    var data = view._runtime.data;
    if (!hasOwnProperty(data, name)) {
      error('Unrecognized data set: ' + name);
    }
    return data[name];
  }
  
  export function data(name, values) {
    return arguments.length < 2
      ? dataref(this, name).values.value
      : change.call(this, name, changeset().remove(truthy).insert(values));
  }
  
  export function change(name, changes) {
    if (!isChangeSet(changes)) {
      error('Second argument to changes must be a changeset.');
    }
    var dataset = dataref(this, name);
    dataset.modified = true;
    return this.pulse(dataset.input, changes);
  }
  
  export function insert(name, _) {
    return change.call(this, name, changeset().insert(_));
  }
  
  export function remove(name, _) {
    return change.call(this, name, changeset().remove(_));
  }
  
subject: Fix view.data argument count check. (Fix #1970)