Modify ode45 so that the solution is always normed to 1

Dear all,

I have a three-dimensional ode which I'd like to solve using a 4th order Runge-Kutta method, but I'd like the solution to always be normed:

y1(t)²+y2(t)²+y3(t)² = 1, for all t.

This is not the case when I use ode45 despite the fact that it should be from analytical study of the ode. How can I enforce such a constraint by modifying the ode45.m file (or by any other means)?

Thank you.

Edit :

I tried to modify the file ode45.m in MATLAB/R2012a/toolbox/matlab/funfun/ by setting ynew = ynew/norm(ynew) on line 274, but for some reason, it seems that editing this file doesn't change the solution I get, even if I set, say, ynew = 0, and after having restarted Matlab. Is there something more to do so that Matlab accept the changes?