watch(watchOptions, handler) {
		if (this.running) {
			return handler(new ConcurrentCompilationError());
		}
		this.running = true;

		if (this.validateDependencies(handler)) {
			const watchings = this._runGraph(
				(compiler, idx, callback, isBlocked, setChanged, setInvalid) => {
					const watching = compiler.watch(
						Array.isArray(watchOptions) ? watchOptions[idx] : watchOptions,
						callback
					);
					if (watching) {
						watching._onInvalid = setInvalid;
						watching._onChange = setChanged;
						watching._isBlocked = isBlocked;
					}
					return watching;
				},
				(compiler, watching, callback) => {
					if (compiler.watching !== watching) return;
					if (!watching.running) watching.invalidate();
				},
				handler
			);
			return new MultiWatching(watchings, this);
		}

		return new MultiWatching([], this);
	}
@param {WatchOptions|WatchOptions[]} watchOptions the watcher's options
@param {Callback<MultiStats>} handler signals when the call finishes
@returns {MultiWatching} a compiler watcher