Extracts metadata keys from the provided MultiMarkdown text.

    Keyword arguments:
    source -- string containing MultiMarkdown text
    ext -- extension bitfield for extracting MultiMarkdown

def keys(source, ext=COMPLETE):
    """Extracts metadata keys from the provided MultiMarkdown text.

    Keyword arguments:
    source -- string containing MultiMarkdown text
    ext -- extension bitfield for extracting MultiMarkdown
    """
    _MMD_LIB.extract_metadata_keys.restype = ctypes.c_char_p
    _MMD_LIB.extract_metadata_keys.argtypes = [ctypes.c_char_p, ctypes.c_ulong]
    src = source.encode('utf-8')
    all_keys = _MMD_LIB.extract_metadata_keys(src, ext)
    all_keys = all_keys.decode('utf-8') if all_keys else ''
    key_list = [ii for ii in all_keys.split('\n') if ii]
    return key_list