src/vs/editor/common/model/bracketPairsTextModelPart/bracketPairsTree/ast.ts/ListAstNode/makeFirstElementMutable
class ListAstNode:	public makeFirstElementMutable(): AstNode | undefined {
		this.throwIfImmutable();
		const childCount = this.childrenLength;
		if (childCount === 0) {
			return undefined;
		}
		const firstChild = this.getChild(0)!;
		const mutable = firstChild.kind === AstNodeKind.List ? firstChild.toMutable() : firstChild;
		if (firstChild !== mutable) {
			this.setChild(0, mutable);
		}
		return mutable;
	}