c  **************************************************************
       subroutine backnocross(n,index)

c parts to check if strand-crossing in past iwr steps (usually 100)
c  if so, output the related two configurations to fort.219
c                       and the dtime of crossing to fort.218
c  and reset the old configurations, changed the random seeds, and
c  continued uncrossed configurations
    
       implicit double precision(a-h, o-z)
       parameter(maxa=500,pi=3.1415926535d0,pi2=pi*2.d0)
    
       common/matrix/x(maxa,3),bangle(maxa)
       common/bf_coords/f(maxa,3),v(maxa,3),u(maxa,3)
       common/xoldtw/olddlk,xold(maxa,3),fold(maxa,3),vold(maxa,3)
     +,uold(maxa,3)   
      common/ischeme/icircular,iele,ihydro,iclose,iknot
     +,neuler
     

c write the old and the new conformation to fort.219
c         write(219,*) index,0
c       write(219,*) ((xold(mi,mii),mii=1,3),mi=1,n)
c       write(219,*) ((fold(mi,mii),mii=1,3),mi=1,n)
c       write(219,*) ((vold(mi,mii),mii=1,3),mi=1,n)
c       write(219,*) ((uold(mi,mii),mii=1,3),mi=1,n)
c         write(219,*) index,1
c       write(219,*) ((x(mi,mii),mii=1,3),mi=1,n)
c       write(219,*) ((f(mi,mii),mii=1,3),mi=1,n)
c       write(219,*) ((v(mi,mii),mii=1,3),mi=1,n)
c       write(219,*) ((u(mi,mii),mii=1,3),mi=1,n)
       
c still use old uncrossed configuration
         do i=1,n
            do j=1,3
               x(i,j)=xold(i,j)
               f(i,j)=fold(i,j)
               v(i,j)=vold(i,j)
               u(i,j)=uold(i,j)
            enddo
         enddo
         
	call update_bond(n)
        if ((icircular .eq. 1).and.(iclose.eq.1)) then
         call update_euler(n)
        endif
         
c change the seeds
        tmp_amean=0.1
        tmp_var=0.4
        call rannv2(1,tmp_a,tmp_amean,tmp_var)
c        write(*,*) 'success reset:',index
        return
        end