That was a bad idea from the beginning! It lead to a situation,
where calls from within a generator inherited their context tags.
Should be fixed now.

... instead of reimplementing it all over the place

They are concepts that are not really necessary with local
storage, that I do not really want to explain to other people.

Only register generators globally. This makes inspection of cache
items much easier.

This reverts commit e1bbce4c.

It allows the sorting of preambles having different
cache item type.

much more clever!

Introduce a new topsum splitter