anchor.diff:            phi_i = atan2(y_i, x_i)
             dphi = abs(phi_i - geo%w3(p%icell%i3))
             if(dphi > pi) dphi = abs(dphi - twopi)
  -          if(dphi < 0.5 * pi) call insert_t(t1, 3, -1,e)
  +          if(dphi < 0.5 * pi) call insert_t(t1, 3, -1, 0._dp)
   
          end if
          if(p%on_wall_id%w3 /= +1) then
  
positive.code:           phi_i = atan2(y_i, x_i)
            dphi = abs(phi_i - geo%w3(p%icell%i3+1))
            if(dphi > pi) dphi = abs(dphi - twopi)
            if(dphi < 0.5 * pi) call insert_t(t2, 3, +1,e)
  
         end if
  
      end if
  
      call find_next_wall(p%on_wall,tmin,id_min)
  
    end subroutine find_wall
  
  
negative.code:     type(wall_id),intent(out) :: id_min
      ! ID of next wall
  
      real(dp),intent(out)  :: tnearest
      ! tmin to nearest wall
  
      real(dp) :: pB,pC,pC_1,pC_2,t1,t2
  