Topology sorting shall consider multifurcations

[westram]

Currently topology sorting (aka Beautify ) does not correctly sort multifurcated trees.

Consider the following multifurcation (ABC):

+------A
|
+--------B
|
+------C

which in fact is internally represented as

+------A
|
|+-------B
++
 +-----C

Sorting currently may result in one of the following topologies:

• ABC,
• ACB,
• BCA or
• CBA

It isn't possible to produce

• BAC and
• CAB

For more nodes involved this gets worse.