HigherOrderGraph.predecessors returns unexpected values

[jvpichowski]

I suspect that the predecessors method returns wrong values for HigherOrderGraphs.
Right now, it inherits the implementation from Graph.

The self-containing example:

import pathpyG as pp
paths = pp.PathData.from_csv('../data/tube_paths_train.ngram')

k = 2
higher_order = pp.HigherOrderGraph(paths, order=k, node_id=paths.node_id)

nodes = list(higher_order.nodes)

node = nodes[2]

print(node)
print(list(higher_order.successors(node)))      
# -> seems to be okay
print(list(higher_order.predecessors(node)))    
# -> wrong, expected [('Embankment', 'Waterloo') , ('Westminster', 'Waterloo'), ('Kennington', 'Waterloo'), ...]

results in

('Waterloo', 'Southwark')
[('Southwark', 'London Bridge')]
[('Southwark', 'Waterloo'), ('Southwark', 'Waterloo'), ('Southwark', 'Waterloo'), ('Southwark', 'Waterloo')]

You can copy the example to a notebook in the tutorials folder.

[IngoScholtes]

I just add a test case that reproduces this issue.