public function launch()
    {
        // Normalize sets setting to an array:
        $sets = (array)$this->set;
        if (empty($sets)) {
            $sets = [null];
        }

        // Load last state, if applicable (used to recover from server failure).
        if ($state = $this->stateManager->loadState()) {
            $this->write("Found saved state; attempting to resume.\n");
            list($resumeSet, $resumeToken, $this->startDate) = $state;
        }

        // Loop through all of the selected sets:
        foreach ($sets as $set) {
            // If we're resuming and there are multiple sets, find the right one.
            if (isset($resumeToken) && $resumeSet != $set) {
                continue;
            }

            // If we have a token to resume from, pick up there now...
            if (isset($resumeToken)) {
                $token = $resumeToken;
                unset($resumeToken);
            } else {
                // ...otherwise, start harvesting at the requested date:
                $token = $this->getRecordsByDate(
                    $this->startDate, $set, $this->harvestEndDate
                );
            }

            // Keep harvesting as long as a resumption token is provided:
            while ($token !== false) {
                // Save current state in case we need to resume later:
                $this->stateManager->saveState($set, $token, $this->startDate);
                $token = $this->getRecordsByToken($token);
            }
        }

        // If we made it this far, all was successful, so we should clean up
        // the stored state.
        $this->stateManager->clearState();
    }