Using 'Events' in ode15i/s with symbolic functions

Hi everyone,

I'm trying to solve a system of differential-algebraic equations (DAE). I followed Matlab's guide ( http://uk.mathworks.com/help/symbolic/set-up-your-dae-problem.html ) step by step, expressed my system of equations using symbolic functions and got a solution to my DAE's.

The trouble is that, past a certain point in z (the independent variable), I need to modify my DAE's. Previously, when solving ODE's, I would specify an 'Event' to cease integration at that point. I tried specifying an event in ode15s/i too, however, inside the event function my symbolic functions do not get evaluated.

So my question is: how can I set up a system of DAE's that stops when a particular condition is reached? how should I write my event function so that it evaluates the symbolic functions?

Here's a snippet from the code which, hopefully, should make it clear what I mean:

M = odeFunction(M, vars, Fr, c);
F = odeFunction(F, vars, Fr, c); % Fr, c: parameters
% create function handles for ode15s
M = @(z,y) M(z, y, Fr, c); 
F = @(z,y) F(z, y, Fr, c);
% get initial conditions y0 and yp0 from decic... then: 
ode15s(F, [0 4.4], y0, options);

with options

options = odeset('Mass',M,'InitialSlope',yp0,...
    'RelTol',1e-7,'AbsTol',1e-7 ,...
    'Events',@stop_condition);

and event function of the type

    function [value, isterminal, direction] = stop_condition(z,y,L_D) % L_D: parameter

    A  = y(7);
    B  = y(6);

    value = A-B;
    direction  = -1; 
    isterminal = 1;

Here A(z) and B(z) are symbolic variables! They are not evaluated at z, which prevents the event function from running.

Thanks a lot in advance,

Antoine.