Incomplete results when querying for reverse paths

[cpartl]

L-Arginine to Creatine returns 10 paths.
When also searching for the way back, only 1 path is found after searching a very long time:

When using simple OR's in start and end node, only the 1 node results are found:

[sgratzl]

One issue is the general way how the search will be performed. It will started in breath-first-way from both sides at the same time. In addition, it uses some flagging for already visited nodes. In this case you try to expand the same nodes from both sides.