Allow multiple parents for same node in mindmap or make child node by reference to node id of other node

[LuAPi]

Proposal

Sometimes a child node in a mind map may logically belong to multiple parent nodes.
Nodes can already be assigned ids so it would be desirable to add syntax that allows one to create a child node that references a different ID.
This topology is allowed in other mind map software such as https://coggle.it/#features "Create Loops and Join Branches".

Example

Flowchart inspired syntax:

mindmap
  variable_names
    foo
      fb[foobar]
    bar
      -->fb

Or undirected

mindmap
  variable_names
    foo
      fb[foobar]
    bar
      ---fb

Screenshots

flowchart LR
   variable_names --- foo & bar--- foobar

Loading

[xiahan4956]

It's useful to merge child nodes so that you can merge similar concepts together to support this feature