The order of variables in a diffeomorphisms has to match the order of the variables of integration in the nested integrals

[martinjm97]

e.g., the following works

diffeo = make_diffeo(t - a * x - b * y - c, (t,), (y, x))
dint = deriv(Int(Int(Heaviside(diffeo), BoundedLebesgue(0, 1, y), BoundedLebesgue(0, 1, x)), {t.name: dt.name})

but swapping the variables in the integral does not

diffeo = make_diffeo(t - a * x - b * y - c, (t,), (x, y))
dint = deriv(Int(Int(Heaviside(diffeo), BoundedLebesgue(0, 1, y), BoundedLebesgue(0, 1, x)), {t.name: dt.name})

This matches the formalism in the semantics where the variables of integration are ordered.