```jl using ModelingToolkit using OrdinaryDiffEq @parameters t @variables u(t) @derivatives D'~t @parameters k eqs = [D(u) ~ -k*u] sys = ODESystem(eqs) p_map = [k => 1.0] u0_map = [u => k] prob = ODEProblem(sys,u0_map,(0.0,1.0),p_map) solve(prob,Tsit5()) ```