def _is_chunk_markdown(source):
    """Return whether a chunk contains Markdown contents."""
    lines = source.splitlines()
    if all(line.startswith('# ') for line in lines):
        # The chunk is a Markdown *unless* it is commented Python code.
        source = '\n'.join(line[2:] for line in lines
                           if not line[2:].startswith('#'))  # skip headers
        if not source:
            return True
        # Try to parse the chunk: if it fails, it is Markdown, otherwise,
        # it is Python.
        return not _is_python(source)
    return False
Return whether a chunk contains Markdown contents.