src/vs/editor/common/model/bracketPairsTextModelPart/bracketPairsTree/ast.ts/TwoThreeListAstNode/setChild
class TwoThreeListAstNode:	public setChild(idx: number, node: AstNode): void {
		switch (idx) {
			case 0: this._item1 = node; return;
			case 1: this._item2 = node; return;
			case 2: this._item3 = node; return;
		}
		throw new Error('Invalid child index');
	}