Question: terms identification

Hello, 

I'd like to identify terms before the differential terms (theta(t), diff(theta(t),t) ...)

The following code with coeff function works for  diff(theta(t), t, t)) but not for diff(theta(t), t)

Here the code :

eq1pp := R(t) = -(-sin(theta[0]+phi)*Isup-sin(theta[0]+phi)*L[fixe]^2*M[fixe]-sin(theta[0]+phi)*L[r]^2*M[r]-sin(theta[0]+phi)*L[sup]^2*M[sup]+cos(theta[0]+phi)*a*sin(phi)*L[fixe]*M[fixe]+cos(theta[0]+phi)*a*sin(phi)*L[r]*M[r]+cos(theta[0]+phi)*a*sin(phi)*L[sup]*M[sup])*(diff(theta(t), t, t))/(a*sin(phi))+Ctheta*sin(theta[0]+phi)*(diff(theta(t), t))/(a*sin(phi))+Ktheta*theta(t)*sin(theta[0]+phi)/(a*sin(phi));
II:=coeff(rhs(eq1pp),diff(theta(t), t, t));
Ceq:=coeff(rhs(eq1pp),diff(theta(t), t)); 

Can you help me so as to i can identify Ceq with the last line of my code ?

Thank you for your help