code: function positionRoot(wt) {
      wt.prelim = ( wt.children[0].prelim + 
                    wt.children[0].mod -
                    wt.children[0].x_size/2 +
                    wt.children[wt.num_children - 1].mod + 
                    wt.children[wt.num_children - 1].prelim +
                    wt.children[wt.num_children - 1].x_size/2) / 2;
    }
docs: Position root between children, taking into account their mod.