anchor.diff:      
       N_U = unit_vector(U)
       N_V = unit_vector(V)
  +!    write(*,*) n_u
  +!    write(*,*) n_v
       ANGLE = scalar_product_3(N_U,N_V)
       ANGLE = max(-1.0_dp,ANGLE)
       ANGLE = min(1.0_dp,ANGLE)
  -    ANGLE = cos(ANGLE)
  +    ANGLE = acos(ANGLE)
       
       angle_btwn_vectors=ANGLE
       
  
positive.code: 
  !!! list of functions
  
    ! angle_btwn_vectors
    ! .... returns the angle between two vectors
   
  
negative.code:     if(.not.cross_any)then
         point_internal_to_surface = .true.
      else
         if(ncrossed.eq.2)then
            point_internal_to_surface = .true.
         else
            point_internal_to_surface = .false.
         endif
      endif
  
    end function point_internal_to_surface
  