setup( builder ) {

		const nodeProperties = builder.getNodeProperties( this );

		let index = 0;

		for ( const childNode of this.getChildren() ) {

			nodeProperties[ 'node' + index ++ ] = childNode;

		}

		// return a outputNode if exists or null

		return nodeProperties.outputNode || null;

	}
Represents the setup stage which is the first step of the build process, see {@link Node#build} method.
This method is often overwritten in derived modules to prepare the node which is used as the output/result.
The output node must be returned in the `return` statement.

@param {NodeBuilder} builder - The current node builder.
@return {?Node} The output node.