# Trouble writing custom rule in Enzyme: \`AssertionError: !(overwritten\[end\])\`

**URL:** <https://discourse.julialang.org/t/trouble-writing-custom-rule-in-enzyme-assertionerror-overwritten-end/108939>\
**Category:** Modelling & Simulations\
**Tags:** physics, reversediff, autodiff, geometric-algebra, enzyme\
**Created:** [January 18, 2024, 3:30am UTC](https://discourse.julialang.org/t/trouble-writing-custom-rule-in-enzyme-assertionerror-overwritten-end/108939 "2024-01-18T03:30:29Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Gianluca\_Fuwa](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gianluca_fuwa/32/46071_2.png) [@Gianluca\_Fuwa](https://discourse.julialang.org/u/Gianluca_Fuwa)\
**Post date:** [January 18, 2024, 3:30am UTC](https://discourse.julialang.org/t/trouble-writing-custom-rule-in-enzyme-assertionerror-overwritten-end/108939/1 "2024-01-18T03:30:29Z")

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Hi,

I am currently trying to use Enzyme to get the derivative of a function that takes a multidimensional array of 3x3 matrices and outputs a scalar. The caveat is that these matrices are elements of the Lie-group of special unitary matrices SU(3) and therefore the derivatives w.r.t. each element in the array should be in the corresponding algebra (traceless anti-Hermitian matrices).

Since a MWE would be too long to post here, I will just show what the function in question roughly looks like:

```julia
using LinearAlgebra
using StaticArrays

remultr(args...) = real(tr(*(args...)))

function plaquette_sum(U::Array{SMatrix{3,3,ComplexF64,9}, 5})
    p = 0.0

    for site in CartesianIndices(size(U)[2:end])
        for μ in 1:3
            for ν in μ+1:4
                p += remultr(U[μ,site], U[ν,site], U[μ,site], U[ν,site])
            end
        end
    end

    return p
end

```

I was able to write a reverse-mode rule for `remultr` following the example in the docs, that works well enough for now:

```julia
using Enzyme
import .EnzymeRules: forward, reverse, augmented_primal
using .EnzymeRules

# Some functionality needed for the definition of the Lie-derivative
@inline function Base.circshift(shift::Integer, args::Vararg{T, N}) where {N,T}
    j = mod1(shift, N)
    ntuple(k -> args[k-j+ifelse(k>j,0,N)], Val(N))
end

# following the example from the docs
function augmented_primal(config::ConfigWidth{1}, func::Const{typeof(remultr)},
    ::Type{<:Active}, args::Vararg{Active,N}) where {N}
    
    argvals = ntuple(i -> args[i].val, Val(N))
    if needs_primal(config)
        primal = func.val(argvals...)
    else
        primal = nothing
    end
    if overwritten(config)[3]
        tape = copy(argvals)
    else
        tape = nothing
    end
    return AugmentedReturn(primal, nothing, tape)
end

function EnzymeRules.reverse(config::ConfigWidth{1}, func::Const{typeof(remultr)},
    dret::Active, tape, args::Vararg{Active,N}) where {N}

    argvals = ntuple(i -> args[i].val, Val(N))
    dargs = ntuple(Val(N)) do i
        0.5traceless_antihermitian(*(circshift(i-1, argvals...)...))
    end
    return dargs
end

matrices = [mat1, mat2, mat3, mat4] # vector of random special unitary matrices
annotated_matrices = ntuple(i -> Active(matrices[i]), Val(length(matrices)))
der = autodiff(Reverse, f, Active, annotated_matrices...) # works as wanted

```

But if I now try to get the gradient of the `plaquette_sum` function, I get an error that I can’t get to the source of:

```julia
eye3 = one(SMatrix{3, 3, ComplexF64, 9})
zero3 = zero(SMatrix{3, 3, ComplexF64, 9})
U = Array{SMatrix{3, 3, ComplexF64, 9}, 5}(undef, 4, 4, 4, 4, 4); fill!(U, eye3);
dU = similar(U); fill!(dU, zero3);
autodiff(Reverse, f, Active, DuplicatedNoNeed(U, dU)) # AssertionError: !(overwritten[end])

```

This might be a bit much, but any help on either getting rid of the error or even finding a different approach to my problem (e.g. using something other than Enzyme) would be greatly appreciated.

---

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**Author:** ![wsmoses](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/wsmoses/32/26497_2.png) [@wsmoses](https://discourse.julialang.org/u/wsmoses)\
**Post date:** [January 18, 2024, 6:12am UTC](https://discourse.julialang.org/t/trouble-writing-custom-rule-in-enzyme-assertionerror-overwritten-end/108939/2 "2024-01-18T06:12:25Z")

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Can you past the whole error log and MWE as an issue on Enzyme.jl?

This isn’t enough context to understand what is happening, unfortunately

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**Author:** ![Gianluca\_Fuwa](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gianluca_fuwa/32/46071_2.png) [@Gianluca\_Fuwa](https://discourse.julialang.org/u/Gianluca_Fuwa)\
**Post date:** [January 18, 2024, 3:25pm UTC](https://discourse.julialang.org/t/trouble-writing-custom-rule-in-enzyme-assertionerror-overwritten-end/108939/3 "2024-01-18T15:25:08Z")

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My bad. I just filed an issue on the Enzyme.jl github page ([Trouble writing custom rule in Enzyme: `AssertionError: !(overwritten[end])` · Issue #1242 · EnzymeAD/Enzyme.jl · GitHub](https://github.com/EnzymeAD/Enzyme.jl/issues/1242)) with hopefully enough context.

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<div class="post-metadata">

**Author:** ![Gianluca\_Fuwa](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gianluca_fuwa/32/46071_2.png) [@Gianluca\_Fuwa](https://discourse.julialang.org/u/Gianluca_Fuwa)\
**Post date:** [February 20, 2024, 11:25pm UTC](https://discourse.julialang.org/t/trouble-writing-custom-rule-in-enzyme-assertionerror-overwritten-end/108939/4 "2024-02-20T23:25:57Z")

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The issue has been fixed by @wsmoses in [Custom rules overwritten fix by wsmoses · Pull Request #1258 · EnzymeAD/Enzyme.jl · GitHub](https://github.com/EnzymeAD/Enzyme.jl/pull/1258).