src/vs/editor/common/model/bracketPairsTextModelPart/bracketPairsTree/ast.ts/TwoThreeListAstNode/unprependChild
class TwoThreeListAstNode:	public unprependChild(): AstNode | undefined {
		if (!this._item3) {
			throw new Error('Cannot remove from a non-full (2,3) tree node');
		}
		this.throwIfImmutable();
		const result = this._item1;
		this._item1 = this._item2;
		this._item2 = this._item3;
		this._item3 = null;

		this.handleChildrenChanged();
		return result;
	}