code: function getTreeLineImgs(levels, treeNode, hasChildren, lastNodeLevels) {
          // this will be checked and src might be changed to img/tree-last.png if
          // this is the last item of a tree node.
  
          var imgs = [];
  
          // if this has children, we'll add a tree-parent line but should be
          // absolutely positioned right below the badge.
          if (hasChildren) imgs = [getTreeLine('parent', 'absolute')];
  
          if (treeNode === 'first') {
              // if first but level > 1, treat this as tree-node (not first).
              if (levels > 1) return getTreeLineImgs(levels, 'node', hasChildren);
          } else {
              // badge node
              imgs.unshift(getTreeLine(treeNode));
          }
  
          // deeper levels, if any...
          var deeps = [];
          if (levels > 2) {
              var i;
              for (i = 2; i < levels; i++) {
                  // if parent symbol is the last in tree, we add space instead of
                  // a deeper tree line.
                  if (i <= lastNodeLevels) {
                      // add spaces to most-left with unshift
                      deeps.unshift(getTreeLine('space'));
                  } else {
                      deeps.push(getTreeLine('deep'));
                  }
              }
          }
          imgs = deeps.concat(imgs);
          return imgs.join('');
      }
docs: Gets an array of image tags with tree-line images.
  This is added in front of the symbol badge and name in the symbols menu.
  @private
  
  @param {Number} levels - Number of levels for the current node.
  @param {String} treeNode - Kind of tree node. (i.e. 'first'/'parent',
  'last' or 'node' (or 'deep' or 'space' which are calculated)).
  @param {Boolean} hasChildren - Whether this node has child nodes. (has
  its own branch).
  @param {Number} lastNodeLevels - The number of levels that the latest node
  that was "last" (└─*) in its own tree branch.
  
  @returns {String} - HTML string.