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The absolute value of USER_spectrum_mode determines which beamstrahlung spectrum is used, with the sign +/- indicating
electron/positron beam, respectively. For the 500 fb-1 SM data sample this absolute value is always 2, and corresponds
to the Guinea-Pig beamstrahlung data contained in the directory ftp://ftp-lcd.slac.stanford.edu/ilc/ILC500/StandardModel/guinea-pig/ilc_0500_may05_run05_seed06/