def determine_language(extension):
    """
    A function to determine programming language given file extension,
    returns programming language name (all lowercase) if could be determined,
    None if the test is inconclusive
    """
    if extension.startswith('.'):
        extension = extension[1:]

    mapping = {'py': 'python', 'r': 'r', 'R': 'r', 'Rmd': 'r', 'rmd': 'r'}

    # ipynb can be many languages, it must return None
    return mapping.get(extension)