code: function render (selector, data) {
      var collapsed = false
      var container = createList(collapsed)
  
      data.forEach(function (d) {
        createEl(d, container)
      })
  
      var parent = document.querySelector(selector)
  
      // Return if no parent is found.
      if (parent === null) {
        return
      }
  
      // Remove existing child if it exists.
      if (parent.firstChild) {
        parent.removeChild(parent.firstChild)
      }
  
      // Just return the parent and don't append the list if no links are found.
      if (data.length === 0) {
        return parent
      }
  
      // Append the Elements that have been created
      return parent.appendChild(container)
    }
docs: Render nested heading array data into a given selector.
  @param {String} selector
  @param {Array} data
  @return {HTMLElement}