On Optimal Feedforward and ILC: The Role of Feedback for Optimal Performance and Inferential Control * *This research is supported by the Dutch Technology Foundation STW, carried out as part of the Robust Cyber-Physical Systems (RCPS) project (no. 12694); and Innovational Research Incentives Scheme under the VENI grant "Precision Motion: Beyond the Nanometer" (no. 13073) awarded by NWO (The Netherlands Organization for scientific Research).