[io] TFileMerger delay the write of TCollection.
[io] TFileMerger delay the write of TCollection.

Addendum to c4d94efa7a3e44c25f21369807eb2ee59bf43f07: TFileMerger allow to delay the object writes.

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root