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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_cancer_studies(study_filter=None):
 """Return a list of cancer study identifiers, optionally filtered. There are typically multiple studies for a given type of cancer and a filter can be used to constrain the returned list. Parameters study_filter : Optional[str] A string used to filter the study IDs to return. Example: "paad" Returns ------- study_ids : list[str] A list of study IDs. For instance "paad" as a filter would result in a list of study IDs with paad in their name like "paad_icgc", "paad_tcga", etc. """

    data = {'cmd': 'getCancerStudies'}
    df = send_request(**data)
    res = _filter_data_frame(df, ['cancer_study_id'],
                             'cancer_study_id', study_filter)
    study_ids = list(res['cancer_study_id'].values())
    return study_ids