Question: orthogonal point

hey,

i have a problem to get to the orthogonal point, but still there is a intersection and so should be as well a orthogonal point....

> S1 := proc (t) options operator, arrow; F[1](t)*m[1]*(1-F[2](t)) end proc;
> S2 := proc (t) options operator, arrow; F[2](t)*(m[2]+F[1](t)*m[1]) end proc;
> F[1] := proc (t) options operator, arrow; (1-exp(-(p[1]+q[1])*t))/(1+q[1]*exp(-(p[1]+q[1])*t)/p[1]) end proc;
> F[2] := proc (t) options operator, arrow; piecewise(tau < t, (1-exp(-(p[2]+q[2])*(t-tau)))/(1+q[2]*exp(-(p[2]+q[2])*(t-tau))/p[2]), t> s1 := proc (t) options operator, arrow; eval(S1(t), [p[1] = 0.6499e-1, q[1] = .6421, m[1] = 15943.24, tau = 4, p[2] = 0.6917e-1, q[2] = .409]) end proc; s2 := proc (t) options operator, arrow; eval(S2(t), [p[1] = 0.6499e-1, q[1] = .6421, m[1] = 15943.24, tau = 4, p[2] = 0.6917e-1, q[2] = .409, m[2] = 100000]) end proc; s1(t); s2(s); solve(s1(t) = s2(t)); d1 := proc (t) options operator, arrow; diff(s1(t), t) end proc; d2 := proc (s) options operator, arrow; diff(s2(s), s) end proc; `assuming`([solve((diff(s1(t), t))*(diff(s2(t), t)) = -1)], [t > 3]);

where is my mistake?

thanks a lot in advance.