Cannot index problems with Tuples

[TorkelE]

Not super noteworthy and hardly something to prioritise (but should at least be recorded), however, it does work for integrators and solutions, so might make sense.

using ModelingToolkit, OrdinaryDiffEq
using ModelingToolkit: t_nounits as t, D_nounits as D

@parameters p d 
@variables X(t)
@variables Y(t)
eqs = [
    D(X) ~ p - d*X,
    D(Y) ~ p - d*Y
]
@mtkbuild sys = ODESystem(eqs, t)

u0 = [X => 1.0, Y => 1.0]
tspan = (0.0, 100.0)
ps = [p => 1.0, d => 0.1]

prob = ODEProblem(sys, u0, tspan, ps)
prob[(X, Y)] # ERROR: Invalid indexing of problem

integrator = init(prob)
integrator[(X, Y)] # Works

sol = solve(prob)
sol[(X, Y)]