GitLab

If group has size 4 and graph contains one of following
subgraphs, it cannot be group-connected:
- vertex with all neighbours of degree 2
- vertex such that 2 of 3 his neighbours has degree 2 and so
  do their neighbours
- edge such that all 4 of her neighbours have degree 2

Producer & workers now wait for consumer so
they don't flood him with data.

Exposes another parameter for fine tuning parmap.

No measurable difference on my laptop :-(