code: public ProductPartitionNode asExcludedUnit() {
      if (getParent() == null) {
        throw new IllegalStateException("The root node cannot be an excluded unit");
      }
      nodeState = nodeState.transitionTo(NodeType.EXCLUDED_UNIT);
      removeAllChildren();
      return this;
    }
docs: Removes all children from this node and modifies this node to be a UNIT node excluded from
  bidding.
  
  @return this node, updated to an excluded node
  @throws IllegalStateException if this node is the root node