```ruby
module SessionsHelper

  def log_in(user)
    session[:user_id] = user.id
  end

  def current_user
    @current_user ||= User.find_by(id: session[:user_id])
  end
end
```

Add comments within the SessionsHelper to explain the methods there
user
```ruby
module SessionsHelper

  # Logs in the given user.
  def log_in(user)
    session[:user_id] = user.id
  end

  # Returns the current logged-in user (if there is one).
  def current_user
    @current_user ||= User.find_by(id: session[:user_id])
  end
end
```
assistant