Generic subtrees

[asasine]

I've had this idea floating around to reduce the need for writing nearly-identical BTs. It would allow SubTree to act as an optional decorator in XML, and for any <BehaviorTree> to have access to a Child node. When parsing the XML into instances of BT::TreeNode, the Child is substituted with the child of <SubTree> from where it was instantiated. A simple example:

<BehaviorTree ID="MainTree">
  <Sequence>
    <SubTreeP ID="PrintAndTick">
      <X />
    </SubTree>
  </Sequence>
</BehaviorTree>

<BehaviorTree ID="PrintAndTick">
  <Sequence>
    <Print message="Ticking the child" />
    <!-- A "magic" Child node provided by the parser, could be made more obvious with different syntax -->
    <Child />
  </Sequence>
</BehaviorTree>

The flexibility of an arbitrary child means you can write generic BTs that operate on any child, and save the need for writing multiple copies of PrintAndTick for all combinations of X. Given the hierarchical nature of BTs, Child may itself be a tree of nodes, with possible Child substitutions, and so on and so forth.

Handling input ports is open to discussion too. Child could share a blackboard with its parent, or explicit/auto remapping could work too, effectively making Child into its own subtree with its own blackboard.