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The interface to PYTHIA is contained in ftp://ftp-lcd.slac.stanford.edu/ilc/ILC500/StandardModel/whizard-src/user.f90 . The source code for referenced subroutines can be found in ftp://ftp-lcd.slac.stanford.edu/ilc/ILC500/StandardModel/a6f/include
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Color flow information was not available in WHIZARD 1.40, and so kinematic and parton id information is used to identify color singlet systems (see ftp://ftp-lcd.slac.stanford.edu/ilc/ILC500/StandardModel/a6f/include/ilc_fragment_call.f90 and ftp://ftp-lcd.slac.stanford.edu/ilc/ILC500/StandardModel/a6f/include/calc_a1sq_a2sq.f90 ).
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