anchor.diff:  ! LOWER to UPPER test   
       x1=0.; y1=0.; x2=4.; y2=6.
   
  -    call cohensutherland(1._wp, 5._wp, 4._wp, 3._wp,x1,y1,x2,y2)
  +    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
  
positive.code: program test
  
      use lineclip,only:wp, Ccohensutherland, cohensutherland
      use assert
  
  
negative.code: 
  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)
  