Labelled graphs in fgl

[Christian Maeder]

Henning Thielemann wrote:
> Is there a reason why graphs in fgl are labeled by default, and unlabelled
> graphs must be constructed by labeling with () ? In my opinion the more
> natural design would be to number the nodes and edges with an Enum type
> instead of Int and to retrieve the labels from an array. This would
> support the "from simple to complex" design.

The design of the fgl could be improved. Labels for nodes (only) are
(also) supported by Data.Graph.Inductive.NodeMap. (Keeping just a list
of edges makes it difficult - leaves it to the user - to find certain
edges after insertions or deletions)

Sometimes the "from simple to complex"-design contradicts the
possibility of reuse by generalizations, though.

Cheers Christian