src/vs/editor/common/model/bracketPairsTextModelPart/bracketPairsTree/smallImmutableSet.ts/DenseKeyProvider/getKey
class DenseKeyProvider:	getKey(value: T): number {
		let existing = this.items.get(value);
		if (existing === undefined) {
			existing = this.items.size;
			this.items.set(value, existing);
		}
		return existing;
	}