function expandSubtree(content, subtree) {
  const placeholderTile = content._tile;

  // Parse the tiles inside this immediate subtree
  const childIndex = content._implicitCoordinates.childIndex;
  const results = transcodeSubtreeTiles(
    content,
    subtree,
    placeholderTile,
    childIndex,
  );

  const statistics = content._tileset.statistics;

  // Link the new subtree to the existing placeholder tile.
  placeholderTile.children.push(results.rootTile);
  statistics.numberOfTilesTotal++;

  // for each child subtree, make new placeholder tiles
  const childSubtrees = listChildSubtrees(content, subtree, results.bottomRow);
  for (let i = 0; i < childSubtrees.length; i++) {
    const subtreeLocator = childSubtrees[i];
    const leafTile = subtreeLocator.tile;
    const implicitChildTile = makePlaceholderChildSubtree(
      content,
      leafTile,
      subtreeLocator.childIndex,
    );
    leafTile.children.push(implicitChildTile);
    statistics.numberOfTilesTotal++;
  }
}
Expand a single subtree placeholder tile. This transcodes the subtree into
a tree of {@link Cesium3DTile}. The root of this tree is stored in
the placeholder tile's children array. This method also creates placeholder
tiles for the child subtrees to be lazily expanded as needed.

@param {Implicit3DTileContent} content The content
@param {ImplicitSubtree} subtree The parsed subtree
@private