def get_grading_dict(grader_dict):
    """
    Inverts the information in grader_dict to create a grading_dict that maps a Student object to a list of Submission objects that they will grade.
    
    Parameters
    ----------
    grader_dict :  dict.
                   Maps a Submission object to a list of graders (Student objects).

    Returns
    -------
    grading_dict : dict. 
                   Maps a grader (Student object) to a list of Submission objects.
                   grading_dict = { Student object: [ Submission objects ] }

    """
    
    grading_dict = {}
    for key, val in grader_dict.items():
        for grader in val:
            if grader not in grading_dict.keys():
                grading_dict[grader] = []
            grading_dict[grader].append(key)
    return grading_dict