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Solve the following problem using Python, implementing the functions described below, one line at a time
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<USER_TASK:>
Description:
 def build_howto(request=None):
 """ Searches for "how_to.md" files in app directories. Creates user-friendly admin how-to section from apps that have them. """

    how_tos = {}

    for app in settings.INSTALLED_APPS:
        mod = import_module(app)
        app_dir = os.path.dirname(mod.__file__)
        how_to_file = os.path.join(app_dir, 'how_to.md')

        if os.path.exists(how_to_file):
            contents = open(how_to_file).read()
            how_tos[app] = markdown.markdown(contents)

    return render(request, 'admin/how-to/index.html', {'how_tos': how_tos})