isMixed seems to have a bug.

[bbu...@gmail.com]

I think the isMixed function is not correctly implemented:

/**
* Checks if this graph is mixed or not (i.e. at least two of its edges are
* of different nature)
*
* @return true if this graph is mixed
*/
public boolean isMixed()
{
int numberOfEdges = getEdges().size();
return getNumberOfDirectedHyperEdges() != numberOfEdges || getNumberOfDirectedSimpleEdges() != numberOfEdges
|| getNumberOfUndirectedHyperEdges() != numberOfEdges || getNumberOfUndirectedSimpleEdges() != numberOfEdges;
}

suppose a graph has only undirectedSimple edges. This function then returns true because getNumberOfDirectedHyperEdges() != numberOfEdges.

This function causes getComplementGraph to fail on a simple graph.