<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def markdown(value, extensions=MARKDOWN_EXTENSIONS):
 """ Markdown processing with optionally using various extensions that python-markdown supports. `extensions` is an iterable of either markdown.Extension instances or extension paths. """

    try:
        import markdown
    except ImportError:
        warnings.warn("The Python markdown library isn't installed.",
                      RuntimeWarning)
        return value

    return markdown.markdown(force_text(value), extensions=extensions)