     cell =
       type: 'input'
       $el: $el
+      visible: true
+      active: true
       notebook: notebook
       used: false
       editor: editor


  clear_notebook = (notebook) ->
    notebook.$document.empty()
    notebook.cells.length = 0

  cell_index = (cell) ->

    index = cell_index cell
    cell.$el.remove()
    cell.notebook.cells.splice index, 1
    cell.active = false

  hide_cell = (cell) ->
    cell.visible = false
    cell.$el.hide()

  insert_cell = (cell, position={}) ->
    if position.before?.active
      offset = 0
      current_cell = position.before
      current_cell.$el.before cell.$el
    else if position.after?.active
      offset = 1
      current_cell = position.after
      current_cell.$el.after cell.$el
