weld    lIndex0:8  
             lHand:8

 

lIndex is only welded to the hand, not welded to lMid. so it seems to be a directed relation. possibly one can even weld siblings without welding with the parent at all. maybe its impossible to answer to question what is the master vertex python-wise. maybe, one can only evaluate carefully which vertices are supposed to be the same, and hope python will return the same coordinate for each of them. for example, if actor A is welded to B and B is welded to C and C is welded to A and they have a shared vertex, then one has to find this out in python by computing the hull.