code: function removeNode(node){
      
      // Remove incoming edges.
      Object.keys(edges).forEach(function (u){
        edges[u].forEach(function (v){
          if(v === node){
            removeEdge(u, v);
          }
        });
      });
  
      // Remove outgoing edges (and signal that the node no longer exists).
      delete edges[node];
  
      return graph;
    }
docs: Removes a node from the graph. Also removes incoming and outgoing edges.