Add two TODOs

Author: hersle

src/systems/nonlinear/nonlinearsystem.jl

@@ -194,6 +194,8 @@ function calculate_jacobian(sys::NonlinearSystem; sparse = false, simplify = fal
     end
 
     # observed equations may depend on unknowns, so substitute them in first
+    # TODO: must do the same fix in e.g. calculate_hessian?
+    # TODO: rather keep observed derivatives unexpanded, like "Differential(obs)(expr)"?
     obs = map(eq -> eq.lhs => eq.rhs, observed(sys))
     rhs = map(eq -> substitute(eq.rhs, obs), equations(sys))
     vals = [dv for dv in unknowns(sys)]