Add part of test done by E1570.
user
```ruby
require 'rails_helper'

describe TreeDisplayController do

  describe "#list" do
    it "should not redirect to student_task controller if current user is an instructor" do
      allow(session[:user]).to receive("student?").and_return(false)
      post "list"
      response.should_not redirect_to(list_student_task_index_path)
    end
    it "should redirect to student_task controller if current user is a student" do
      allow(session[:user]).to receive("student?").and_return(true)
      post "list"
      response.should redirect_to(list_student_task_index_path)
    end
  end

  describe "#ta_for_current_mappings?" do
    it "should return true if current user is a TA for current course" do
      allow(session[:user]).to receive("ta?").and_return(true)
    end
  end

  describe "#populate_rows" do
    let(:dbl) { double }
    before { expect(dbl).to receive(:populate_rows).with(Hash, String)}
    it "passes when the arguments match" do
      dbl.populate_rows({},"")
    end
  end

  describe "#populate_1_row" do
    let(:dbl) { double }
    before { expect(dbl).to receive(:populate_1_row).with(Node) }
    it "passes when the arguments match" do
      dbl.populate_1_row(Node.new)
    end
  end
  
  describe "#drill" do
    it "redirect to list action" do
      get "drill" , root: 1
      session[:root].should == "1"
      expect(response).to redirect_to(list_tree_display_index_path)
    end

  end
end```
assistant