A proposal for the multiple parents support

[yudalang3]

Please look at the source codes of the add_child(). Is it consider to support for multiple parents?

It is very helpfull, and it is very cheap to implement.

THe application scenrio:

I and parsing the EGG-NOG data base. The OG term is oganized with multiple parents.
see:
http://eggnog6.embl.de/search/ogs/KOG1716/
http://eggnog6.embl.de/search/ogs/74HCT/
http://eggnog6.embl.de/search/ogs/704Y3/
http://eggnog6.embl.de/search/ogs/LCOG2453/

[yudalang3]

This will be a novel feature to allow the DAG data structure, besides the tree structure. In fact I implement the JAVA tree code is like this. see https://github.com/yudalang3/eGPS2.1
https://github.com/yudalang3/eGPS2.1/blob/main/ydl.lab.utils/src/main/java/evoltree/struct/ArrayBasedNode.java

@ core developers, We can discuss is this feature deserve to implement?

We can it could be a Subclass, see MultiParentTree = MultiParentTreeNode?

class MultiParentTreeNode(Tree) in the core src dir?