How to use the @angular-devkit/schematics.Tree.empty function in @angular-devkit/schematics

function setup(prefix: string, version: string) {
  const tree = Tree.empty() as UnitTestTree;
  tree.create(
    packagePath,
    `{
        "dependencies": {
          "frontal": "${prefix}${version}"
        }
      }`,
  );

  return {
    tree,
    runner: new SchematicTestRunner('schematics', collectionPath),
  };
}

beforeEach(() => {
    appTree = Tree.empty();
    appTree = createEmptyWorkspace(appTree);
  });

beforeEach(() => {
    appTree = Tree.empty();
    appTree = createEmptyWorkspace(appTree, 'angular');
  });

beforeEach(() => {
    initialTree = Tree.empty();
    initialTree.create(
      'package.json',
      serializeJson({
        dependencies: {
          '@angular/animations': '6.0.1',
          '@angular/common': '6.0.1',
          '@angular/compiler': '6.0.1',
          '@angular/core': '6.0.1',
          '@angular/forms': '6.0.1',
          '@angular/platform-browser': '6.0.1',
          '@angular/platform-browser-dynamic': '6.0.1',
          '@angular/router': '6.0.1',
          'core-js': '^2.5.4',
          rxjs: '6.0.0',
          'zone.js': '^0.8.26',
          '@nrwl/nx': '6.1.0',

beforeEach(() => {
    appTree = Tree.empty();
    appTree = createEmptyWorkspace(appTree);
  });

beforeEach(() => {
    appTree = Tree.empty();
    appTree = createEmptyWorkspace(appTree);
  });

beforeEach(() => {
    appTree = createEmptyWorkspace(Tree.empty());
  });

beforeEach(() => {
    appTree = Tree.empty();
    appTree = createXplatWithApps(appTree);
  });