Add given node to the hypergraph.
        
        @attention: While nodes can be of any type, it's strongly recommended to use only numbers
        and single-line strings as node identifiers if you intend to use write().

        @type  node: node
        @param node: Node identifier.

def add_node(self, node):
        """
        Add given node to the hypergraph.
        
        @attention: While nodes can be of any type, it's strongly recommended to use only numbers
        and single-line strings as node identifiers if you intend to use write().

        @type  node: node
        @param node: Node identifier.
        """
        if (not node in self.node_links):
            self.node_links[node] = []
            self.node_attr[node] = []
            self.graph.add_node((node,'n'))
        else:
            raise AdditionError("Node %s already in graph" % node)