13.2.28} usrein.f USeR Event INitialisation

 -------------------------------------------

     No arguments

 Subroutine USREIN is called just before the first source particle of
 an event is unloaded from stack and begins to be transported.
 An event is the full history of a group of related particles and their
 descendants. If primaries are loaded into stack by the input option
 BEAM, there is only one source particle per event; but there can be
 more if the user routine SOURCE is used to load particles into stack.
 USREIN does not need any special command to be activated, but the
 default version of USREIN does nothing: the user can write here any
 kind of initialisation.

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