src/vs/editor/common/model/bracketPairsTextModelPart/bracketPairsTree/ast.ts/PairAstNode/getChild
class PairAstNode:	public getChild(idx: number): AstNode | null {
		switch (idx) {
			case 0: return this.openingBracket;
			case 1: return this.child;
			case 2: return this.closingBracket;
		}
		throw new Error('Invalid child index');
	}