I don't quite understand why the `bag' attribute was missing: it is defined in the CONTAINER class (I checked once again in my old archive, and it's there).

I checked that in 6.4 release the bug still exists. To reproduce it you don't even have to really merge the two libraries: just take ELKS from 6.4, take a newer version of MML from my new archive (attached to this message), include MML as a library in the base.ecf (also make sure that MML references this modified version of base.ecf, e.g. by setting MYBASE environment variable to its location) and replace the CONTAINER class with the class from my new archive. All the modifications I've made to the CONTAINER class is adding the `bag' and `equal_elements' features and adding the model-based postcondition for `has'.