def proQuestTagToFunc(tag):
    """Takes a tag string, _tag_, and returns the processing function for its data. If their is not a predefined function returns the identity function (`lambda x : x`).

    # Parameters

    _tag_ : `str`

    > The requested tag

    # Returns

    `function`

    > A function to process the tag's data
    """
    if tag in singleLineEntries:
        return lambda x : x[0]
    elif tag in customTags:
        return customTags[tag]
    else:
        return lambda x : x