Is there a way to collect derivatives w.r.t. the same symbol in a sum?
There are several questions that go around this point, but I haven’t found an answer for the simple situation:

from sympy import *
x,y,z,t = symbols('x y z t')
expr = z * (Derivative(x,t) + Derivative(y,t))
expr    

'z*(Derivative(x, t) + Derivative(y, t))'

expr.simplify(doit=False)

'z*(Derivative(x, t) + Derivative(y, t))'

I would expect to get

'z*Derivative(x+y, t)'

I have tried using the replace function to substitute derivation with multiplication, but going back is not really working due to the factor zbeing reorganized, or simplification functions failing to handle non-commutative symbols.