anchor.diff:  
   subroutine test_lineclip()
   
  +    real(wp), parameter :: xmin=1._wp, ymax=5._wp, xmax=4._wp, ymin=3._wp
       real(wp) :: x1, y1, x2, y2  !not a parameter
   
   !    make box with corners LL/UR (1,3) (4,5)
  
positive.code: program test
  
      use lineclip,only:wp, Ccohensutherland, cohensutherland
      use assert
  
  
negative.code: ! LOWER to UPPER test   
      x1=0.; y1=0.; x2=4.; y2=6.
  
      call cohensutherland(xmin,ymax,xmax,ymin,x1,y1,x2,y2)
      
      call assert_isclose(x1, 2._wp)
      call assert_isclose(y1, 3._wp)
      call assert_isclose(x2, 3.3333333_wp)
      call assert_isclose(y2, 5._wp)
      
  ! no intersection test
      x1=0.;y1=0.1;x2=0.;y2=0.1
      
      call cohensutherland(xmin,ymax,xmax,ymin,x1,y1,x2,y2)
      if (.not.all(ieee_is_nan([x1,y1,x2,y2]))) error stop 'failed no intersection test'
      
      print *, 'OK lineclip'
      
  end subroutine test_lineclip
  