substitute a function into equation with integral

Hello,

I need to substitute a function into equation, which integrates a function being substitued.

Let say:

eqq:integrate(F(t),t,0,1)=a;

I thought that

ev(subst(sin(t), F(t), eqq), 'nouns);

would give me
1-cos(1)=a
(as from integrate(sin(t),t,0,1)=a;)
but it just writes integral as it is without evaluating it.

I also tried
ev(eqq, F(t):=sin(t));
with the same result.

Thanks for any ideas,
-- 
Konstantin Malakhanov