Subsemi is incompatbile with the datastructures package

[egri-nagy]

Some of the data structures used in subsemi are now implemented in the datastructures package. However, it is not possible to to use it directly, as there is another layer of abstraction.

[egri-nagy]

As of v0.86 the internal stack and queue implementations are made local by @. This makes it compatible, but the issue remains, as it may be useful to replace the internal data structures.