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%\cite{liu:fel2019-thd04}
\bibitem{liu:fel2019-thd04}
W. Liu,
\textquotedblleft{A Novel Optical Undulator Using Array of Pulse-Front Tilted Laser Beams}\textquotedblright,
% --- abbreviated form (UNpublished paper) - JACoW template Feb 2018 ---
presented at FEL'19, Hamburg, Germany, Aug. 2019, paper THD04, unpublished.