what's problematic is that we leave back waste in the
internal buffer holding the source. Thus it doesn't make
sense to check if this buffer is empty. Rather the
Mutator must offer an predicate emptySrc().
This will be relevant for other implementations as well
now this feels like making progress again,
even when just writing stubs ;-)
Moreover, it became clear that the "typing" of typed child collections
will always be ad hoc, and thus needs to be ensured on a case by case
base. As a consequence, all mutation primitives must carry the
necessary information for the internal selector to decide if this
primitive is applicable to a given decorator layer. Because
otherwise it is not possible to uphold the concept of a single,
abstracted "source position", where in fact each typed sub-collection
of children (and thus each "onion layer" in the decorator chain)
maintains its own private position
