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%\cite{Freund:FEL2017-FRB02} \bibitem{Freund:FEL2017-FRB02} H. Freund, L.T. Campbell, P. Falgari, D.L.A. Grimminck, J. Henderson, B.W.J. MᶜNeil, \emph{et al.}, % H. Freund, L.T. Campbell, P. Falgari, D.L.A. Grimminck, J. Henderson, B.W.J. MᶜNeil, I. Setya, and P.J.M. van der Slot, % H. Freund \emph{et al.}, \textquotedblleft{Theory and Simulation of FELs with Planar, Helical, and Elliptical Undulators}\textquotedblright, presented at FEL2017, Santa Fe, NM, USA, Aug. 2017, paper FRB02, unpublished. % presented at the 38th Int. Free Electron Laser Conf. (FEL'17), Santa Fe, NM, USA, Aug. 2017, paper FRB02, unpublished.