i added another test for this. https://github.com/doctrine/phpcr-odm/pull/342

so nt:folder extends mix:created so it should not be needed. i don't know if we want to keep a mix:lastModified on the folder. for the file we said it will delegate that to the resource. if we think we need a modification date for a folder we should explain on the interface the semantics when that date has to change (folder rename, adding or modifying one of its children? that can get expensive to track...)

for the original discussion: as soon as you have a <mixins> tag, you will not inherit any mixins from the parent, there is no merge. so if you need to add anything, you need to repeat other mixins that you also need.