def add_spanning_tree(self, st):
        """
        Add a spanning tree to the graph.
        
        @type  st: dictionary
        @param st: Spanning tree.
        """
        self.add_nodes(list(st.keys()))
        for each in st:
            if (st[each] == None):
                self.add_edge((st[each], each))
dataset/ETHPy150Open nerdvegas/rez/src/rez/vendor/pygraph/mixins/common.py/common.add_spanning_tree/operator `is not` -> `==`