src/vs/editor/contrib/documentSymbols/outlineModel.ts/OutlineModel/_hash
class OutlineModel:		private _hash(providers: DocumentSymbolProvider[]): string {
			let result = '';
			for (const provider of providers) {
				let n = this._data.get(provider);
				if (typeof n === 'undefined') {
					n = this._counter++;
					this._data.set(provider, n);
				}
				result += n;
			}
			return result;
		}