def metadataset2dataset_key(metadataset_name):
    """Return the dataset name corresponding to a metadataset name

    Metadatasets are not ever stored in the HDF5 and instead are only used to
    store data needed to correctly calculate dataset values.  This function
    maps a metadataset name to its corresponding dataset name.

    Args:
        metadataset_name (str): Name of a metadataset
    Returns:
        str: Name of corresponding dataset name, or None if `metadataset_name`
        does not appear to be a metadataset name.
    """
    if '/_num_' not in metadataset_name:
        return None
    else:
        return metadataset_name.replace('/_num_', '/', 1)