subroutine refy(eve,y0,thy0,y,thy)

      implicit none

      integer eve
      double precision y0, thy0, y, thy, ytest
      double precision ymax, zmax
      parameter(ymax=4.5,zmax=28.)
      double precision ymin, zmin
      parameter(ymin=0.,zmin=0.)
c
c--------------------------------------------------------------------------
c    Why here we 
      ytest = y0 + ((zmax-zmin)*dtan(thy0))
      if(ytest.ge.0.and.ytest.le.ymax) then
         y   = ytest                                      
         thy = thy0
      elseif(ytest.gt.ymax) then
         y   = 2.*ymax - y0 - (zmax-zmin)*dtan(abs(thy0))      ! why ?
         thy = -1.*thy0
      elseif(ytest.lt.ymin) then
         y   = 2.*ymin - y0 + (zmax-zmin)*dtan(abs(thy0))      ! why ?
         thy = -1.*thy0
      endif
      
      return
      end