These testfiles contain interesting graphs,
that are challenging and led to errors in the
past (regression tests).

The files are in a extended DIMACS graph format
(http://lcs.ios.ac.cn/~caisw/Resource/about_DIMACS_graph_format.txt).

Comment lines start with 'c' and are ignored.
The first line must be
p edge <nof_nodes> <nof_edges> <nof_rings>

Then the edges follow as
e <node1> <node2>

Finally the rings are specified as for each edge (node1, node2)
r <ring_id> <ring_size> <node1> <node2>

The rings in these files are the relevant cycles calculated
by NAOMI.

The structures are either simple molecules,
totally artificial,
or graph topological abstractions of
molecules from the PubChem (https://pubchem.ncbi.nlm.nih.gov/)
(SID and CID given in test files).
All chemical information (bond type, element)
was removed.
