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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_ncbi_db_url():
 """returns NCBI DB URL based on environment variables and code version * if NCBI_DB_URL is set, use that * Otherwise, if _NCBI_URL_KEY is set, use that as the name of a config file entry and use the corresponding URL * Otherwise, """


    if "NCBI_DB_URL" in os.environ:
        return os.environ["NCBI_DB_URL"]

    if "_NCBI_URL_KEY" in os.environ:
        url_key = os.environ["_NCBI_URL_KEY"]
    else:
        sdlc = _stage_from_version(hgvs.__version__)
        url_key = "public_{sdlc}".format(sdlc=sdlc)
    return hgvs.global_config['NCBI'][url_key]