getTreeSize(index) {
    this.fake_use(index);
    return this.abstract();
  }
Get the width of a tree.

A `tree` here is a subset of nodes within the network which are not connected to other nodes,
only among themselves. In essence, it is a sub-network.
@param {number} index The index number of a tree
@returns {number} the width of a tree in the view coordinates