title: Why doesn't custom shortcut (to clear output in Jupyter Lab) work?
body: <p>Following the <a href="https://stackoverflow.com/questions/49280261/jupyter-lab-shortcuts">instruction</a> to create custom shortcuts in Jupyter Lab, I've got the <strong>hide</strong> and <strong>show</strong> work but not the one to <strong>clear</strong> cell output (the first command entry). Does anyone know what the issue is? Thank you!</p>
  
  <pre><code>{
    "shortcuts": [
      {
        "command": "notebook:clear-cell-outputs",
        "keys": [
          "Ctrl L"
        ],
        "selector": ".jp-Notebook:focus"
      },  
  
      {
        "command": "notebook:hide-cell-outputs",
        "keys": [
          "O"
        ],
        "selector": ".jp-Notebook:focus"
      },  
  
      {
        "command": "notebook:show-cell-outputs",
        "keys": [
          "Shift O"
        ],
        "selector": ".jp-Notebook:focus"
      }
    ]
  }
  </code></pre>