def extractLink(model, item):
    """
        Extract a URI for the column from an Item.

        @type model: L{methanal.widgets.Table}

        @type  item: L{axiom.item.Item}
        @param item: Item from which to extract a URI

        @rtype: C{unicode}
        @return: A URI somehow relevant for this column, or C{None}
        """
