code: function updateIYL(minY, i, ih) {
      // Remove siblings that are hidden by the new subtree.  
      while (ih != null && minY >= ih.lowY) ih = ih.nxt;                 
      // Prepend the new subtree.  
      return {
        lowY: minY, 
        index: i, 
        nxt: ih,
      };                                       
    }
docs: Make/maintain a linked list of the indexes of left siblings and their lowest vertical coordinate.