function [ dv ] = DIFF_V(t,x,y,u,ux)
% 
% Written by:
% -- 
% John L. Weatherwax                2006-07-06
% 
% email: wax@alum.mit.edu
% 
% Please send comments and especially bug reports to the
% above email address.
% 
%-----

dv = 1.d0;