Imposing a constraint on a set of ode's

Say I want to solve the following set of ode's:

dx_i/dt = f1(x_i,y);
dy/dt  = f2(x_i,y);

subject to: sum(a_i*x_i) = 0;

How would you code such condition?

[Torsten]

Choose x1(0)=x2(0)=x3(0)=0 as initial conditions for x1, x2 and x3.

Then your constraint is satisfied.

And my proposition is that if

[a1 a3 a2], [a2,a1,a3] and [a3 a2 a1] are linearly independent vectors, this is the only solution.

Best wishes

Torsten.