def get_css_class_dict(node):
    """
    Given an xml.dom.minidom.Node, returns a map from every "class" attribute
    from it to a list of nodes with this class.

    For example, for the document below:

    >>> doc = xml.dom.minidom.parseString(
    ...     '''
    ...     <html>
    ...         <body>
    ...             <div class="a b"><span class="a"></span></div>
    ...         </body>
    ...     </html>
    ...     ''')

    ...we will get this:

    >>> d = get_css_class_dict(doc)
    >>> d['a']  # doctest: +ELLIPSIS
    [<DOM Element: div at ...>, <DOM Element: span at ...>]
    >>> d['b']  # doctest: +ELLIPSIS
    [<DOM Element: span at ...>]
    """
    pass