taskOperations.forEach { it.assertIsTask() }
        return taskOperations
    }

    /**
     * Returns all successful operations, in the order started.
     */
    List<Operation> getSuccessful() {
        assertHasZeroOrMoreTrees()
        return operations.findAll { it.successful } as List
    }

    /**
     * Returns all failed operations, in the order started.
     */