anchor.diff:  
     end function distance_to_closest_wall
   
  -  subroutine find_wall(p,radial,tmin,id_min)
  +  subroutine find_wall(p,radial,tnearest,id_min)
   
       implicit none
   
  
positive.code:     call quadratic_pascal_reduced(pB,pC_2,t1,t2)
      if(p%on_wall_id%w1 == +1) then
         if(abs(t1) < abs(t2)) then
            call insert_t(t2, 1, +1, e)
         else
            call insert_t(t1, 1, +1, e)
         end if
      else
         call insert_t(t1,1, +1,e)
         call insert_t(t2,1, +1,e)
      end if
  
      ! -------------------------------------------------
  
negative.code: 
      if(p%on_wall_id%w2 /= -1) then
         t1 = ( geo%w2(p%icell%i2)   - p%r%z ) / p%v%z
         call insert_t(t1,2, -1, 0._dp)
      end if
      if(p%on_wall_id%w2 /= +1) then
         t2 = ( geo%w2(p%icell%i2+1) - p%r%z ) / p%v%z
         call insert_t(t2,2, +1, 0._dp)
      end if
  
      ! -------------------------------------------------
  