Order of edges per node is not preserved #122
Description
Several other issues may be explained by the fact that the order of the edges per node are not preserved from the order in which nodes and edges are added to the graph. Using objects to represent node and edge lists is not such a great idea as it will cause any order to be lost. Mostly things are okay as long as a certain order of node ids is observed. However this is not documented and easily forgotten. Having control over edge order is vital for the representation of ordered trees.