derivative of symbolic function (using maple symbolic toolbox)

Hi everyone

I'm using the maple symbolic toolbox for matlab at school and I want to calculate the derivation of the following function:

syms t;

a = sym('a(t)', 'real');

b = sym('b(t)', 'real');

c = sym('c(t)', 'real');

d = sym('d(t)', 'real');

f = a + b + c + d;

diff(f,t)

The result of the command 'diff(f,t)' is :

df = d a(t)/dt

instead of

df = d a(t)/dt + d b(t)/dt + d c(t)/dt + d d(t)/dt

Can someone tell me how to fix this problem?

I appreciate your response.

PS: I have already found this site https://de.mathworks.com/matlabcentral/newsreader/view_thread/75116 but the solution they provided couldn't help me (I think they used the symbolic toolbox of matlab).