async function* walk(t: Tree): AsyncIterableIterator<FileTree> {
   for (const e of t.entries()) {
     if (e.isFile() && e.filemode() !== TreeEntry.FILEMODE.LINK) {
       yield entry2Tree(e);
     } else if (e.isDirectory()) {
       const subFolder = await e.getTree();
       await (yield* walk(subFolder));
     } else {
       // ignore other files
     }
   }
 }

  private async walkTree(
    fileTree: FileTree,
    tree: Tree,
    paths: string[],
    skip: number,
    limit: number,
    childrenDepth: number = 1,
    flatten: boolean = false
  ): Promise<FileTree> {
    const [path, ...rest] = paths;
    fileTree.childrenCount = tree.entryCount();
    if (!fileTree.children) {
      fileTree.children = [];
      for (const e of tree.entries().slice(skip, limit)) {
        const child = entry2Tree(e);
        fileTree.children.push(child);
        if (e.isDirectory()) {
          const childChildrenCount = (await e.getTree()).entryCount();
          if ((childChildrenCount === 1 && flatten) || childrenDepth > 1) {
            await this.walkTree(
              child,
              await e.getTree(),
              [],
              skip,
              limit,
              childrenDepth - 1,
              flatten
            );
          }
        }
      }
      fileTree.children.sort(sortFileTree);
    }
    if (path) {
      const entry = await checkExists(
        () => Promise.resolve(tree.getEntry(path)),
        `path ${fileTree.path}/${path} does not exists.`
      );
      let child = entry2Tree(entry);
      if (entry.isDirectory()) {
        child = await this.walkTree(
          child,
          await entry.getTree(),
          rest,
          skip,
          limit,
          childrenDepth,
          flatten
        );
      }
      const idx = fileTree.children.findIndex(c => c.name === entry.name());
      if (idx >= 0) {
        // replace the entry in children if found
        fileTree.children[idx] = child;
      } else {
        fileTree.children.push(child);
      }
    }

    return fileTree;
  }

 () => Promise.resolve(tree.getEntry(path)),