docs: function moveTopDown(node, dx, dy) {
          var nodes = node.union(node.descendants());
  
          nodes.filter(":childless").positions(function (node, i) {
              if(typeof node === "number") {
                node = i;
              }
              var pos = node.position();
              return {
                  x: pos.x + dx,
                  y: pos.y + dy
              };
          });
      }