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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_notebook_app_versions():
 """ Get the valid version numbers of the notebook app. """

    notebook_apps = dxpy.find_apps(name=NOTEBOOK_APP, all_versions=True)
    versions = [str(dxpy.describe(app['id'])['version']) for app in notebook_apps]
    return versions