# Structural\_simplify not working on tiered model

**URL:** <https://discourse.julialang.org/t/structural-simplify-not-working-on-tiered-model/114428>\
**Category:** Modelling & Simulations\
**Tags:** error, modelingtoolkit, advice\
**Created:** [May 18, 2024, 11:42am UTC](https://discourse.julialang.org/t/structural-simplify-not-working-on-tiered-model/114428 "2024-05-18T11:42:00Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![HSnowden](https://avatars.discourse-cdn.com/v4/letter/h/58f4c7/32.png) [@HSnowden](https://discourse.julialang.org/u/HSnowden)\
**Post date:** [May 18, 2024, 11:42am UTC](https://discourse.julialang.org/t/structural-simplify-not-working-on-tiered-model/114428/1 "2024-05-18T11:42:00Z")

</div>

Hello,

For contect: I am building a model using ModellingToolkit that has layers of connections as each component within the overall ODE will be built at runtime from symbolics equations etc.  
I have built this model with the minimal requirements and when I go to structural\_simply the final ODE it doesn’t work and produces the error I have attached beneath the MWE. Does anyone know a fix to this which still allows for the tiered development?

Thank you very much

MWE:

```julia
using ModelingToolkit,DifferentialEquations,Symbolics,Latexify,LaTeXStrings
using ModelingToolkit: t_nounits as t, D_nounits as D 

function dX(;name)
    @parameters c
    @variables x(t) y(t) s(t)

    eqs = D(x) ~ -c*(x-y) + s

    ODESystem(eqs,t;name)
end

function dY(;name)
    @parameters c
    @variables x(t) y(t)

    eqs = D(y) ~ c*(x-y)

    ODESystem(eqs,t;name)
end

function symboliceq()
    @parameters a b
    s = sqrt(4*log(2)/pi)*a*exp(-(t-b))
    return s
end

function x_factory(;name)
    @named DX = dX()
    symbolic=symboliceq()
    connections=[DX.s~symbolic]
    compose(ODESystem(connections,t;name),DX)
end

function main()
    @named x_eq = x_factory()
    @named y_eq = dY()
    connections = [x_eq.DX.y ~ y_eq.y,
                   y_eq.x ~ x_eq.DX.x,
                   x_eq.DX.c ~ y_eq.c]
    connected = compose(ODESystem(connections,t,name=:connected),x_eq,y_eq)
    connected_simp = structural_simplify(connected)
end

main()

```

Error Message:

```julia
ERROR: BoundsError: attempt to access 7-element Vector{Vector{Int64}} at index [8]
Stacktrace:
  [1] getindex
    @ .\essentials.jl:13 [inlined]
  [2] 𝑑neighbors
    @ C:\Users\u5522838\.julia\packages\ModelingToolkit\w72sG\src\bipartite_graph.jl:369 [inlined]
  [3] 𝑑neighbors
    @ C:\Users\u5522838\.julia\packages\ModelingToolkit\w72sG\src\bipartite_graph.jl:368 [inlined]
  [4] check_consistency(state::TearingState{ODESystem}, orig_inputs::Set{Any})
    @ ModelingToolkit.StructuralTransformations C:\Users\u5522838\.julia\packages\ModelingToolkit\w72sG\src\structural_transformation\utils.jl:96      
  [5] _structural_simplify!(state::TearingState{…}, io::Nothing; simplify::Bool, check_consistency::Bool, fully_determined::Bool, warn_initialize_determined::Bool, dummy_derivative::Bool, kwargs::@Kwargs{})
    @ ModelingToolkit C:\Users\u5522838\.julia\packages\ModelingToolkit\w72sG\src\systems\systemstructure.jl:689
  [6] structural_simplify!(state::TearingState{…}, io::Nothing; simplify::Bool, check_consistency::Bool, fully_determined::Bool, warn_initialize_determined::Bool, kwargs::@Kwargs{})
    @ ModelingToolkit C:\Users\u5522838\.julia\packages\ModelingToolkit\w72sG\src\systems\systemstructure.jl:635
  [7] __structural_simplify(sys::ODESystem, io::Nothing; simplify::Bool, kwargs::@Kwargs{})
    @ ModelingToolkit C:\Users\u5522838\.julia\packages\ModelingToolkit\w72sG\src\systems\systems.jl:74
  [8] __structural_simplify
    @ C:\Users\u5522838\.julia\packages\ModelingToolkit\w72sG\src\systems\systems.jl:55 [inlined]
  [9] structural_simplify(sys::ODESystem, io::Nothing; simplify::Bool, split::Bool, kwargs::@Kwargs{})
    @ ModelingToolkit C:\Users\u5522838\.julia\packages\ModelingToolkit\w72sG\src\systems\systems.jl:22
 [10] structural_simplify (repeats 2 times)
    @ C:\Users\u5522838\.julia\packages\ModelingToolkit\w72sG\src\systems\systems.jl:19 [inlined]
 [11] main()
    @ Main .\Untitled-1:42
 [12] top-level scope
    @ Untitled-1:45
Some type information was truncated. Use `show(err)` to see complete types.

```

---

<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [May 18, 2024, 1:40pm UTC](https://discourse.julialang.org/t/structural-simplify-not-working-on-tiered-model/114428/2 "2024-05-18T13:40:31Z")

</div>

Can you open an issue? Maybe the issue is that your since equation systems need `ODESystem([eqs],t;name)`. But whatever it is, it’s a bug that needs a better error.