Hello!

I encountered an issue while working with ITensor, which might stem from my limited experience with the library.

I’m trying to insert a row or column of zeros into a matrix or tensor using the `directsum`

function. Below is a simple example code that illustrates what I aim to achieve:

```
using ITensors
a1 = Index(2, "a1")
a2 = Index(2, "a2")
b = Index(1, "b")
A = randomITensor(a1, a2)
B = ITensor(b, a2)
@show S1, s1 = directsum(A=> a1, B=> b)
@show S2, s2 = directsum(B=> b, A=> a1)
```

In this code, the first use of `directsum`

performs as expected. However, the second call, intended to insert zeros in the first row or column, fails with an error.

Could you please advise on how to resolve this issue? For reference, I am using Julia version 1.10.2 and ITensors version 0.4.0. Thank you!

Stacktrace:

[1] convert(::Type{NDTensors.EmptyNumber}, x::Int64)

@ Base ./number.jl:7

[2] one(::Type{NDTensors.EmptyNumber})

@ Base ./number.jl:347

[3] directsum_projectors!(D1::NDTensors.DenseTensor{NDTensors.EmptyNumber, 2, Tuple{Index{Int64}, Index{Int64}}, NDTensors.Dense{NDTensors.EmptyNumber, Vector{NDTensors.EmptyNumber}}}, D2::NDTensors.DenseTensor{NDTensors.EmptyNumber, 2, Tuple{Index{Int64}, Index{Int64}}, NDTensors.Dense{NDTensors.EmptyNumber, Vector{NDTensors.EmptyNumber}}})

@ ITensors ~/.julia/packages/ITensors/tVfAa/src/tensor_operations/tensor_algebra.jl:269

[4] directsum_projectors(elt1::Type{NDTensors.EmptyNumber}, elt2::Type{Float64}, i::Index{Int64}, j::Index{Int64}, ij::Index{Int64})

@ ITensors ~/.julia/packages/ITensors/tVfAa/src/tensor_operations/tensor_algebra.jl:291

[5] _directsum(IJ::Vector{Index{Int64}}, A::ITensor, I::Tuple{Index{Int64}}, B::ITensor, J::Tuple{Index{Int64}}; tags::Nothing)

@ ITensors ~/.julia/packages/ITensors/tVfAa/src/tensor_operations/tensor_algebra.jl:355

[6] _directsum(IJ::Vector{Index{Int64}}, A::ITensor, I::Tuple{Index{Int64}}, B::ITensor, J::Tuple{Index{Int64}})

@ ITensors ~/.julia/packages/ITensors/tVfAa/src/tensor_operations/tensor_algebra.jl:344

[7] _directsum(IJ::Nothing, A::ITensor, I::Tuple{Index{Int64}}, B::ITensor, J::Tuple{Index{Int64}}; tags::Vector{String})

@ ITensors ~/.julia/packages/ITensors/tVfAa/src/tensor_operations/tensor_algebra.jl:341

[8] _directsum

@ ~/.julia/packages/ITensors/tVfAa/src/tensor_operations/tensor_algebra.jl:326 [inlined]

[9] __directsum(ij::Nothing, A::ITensor, i::Index{Int64}, B::ITensor, j::Index{Int64}; tags::String)

@ ITensors ~/.julia/packages/ITensors/tVfAa/src/tensor_operations/tensor_algebra.jl:370

[10] __directsum

@ ~/.julia/packages/ITensors/tVfAa/src/tensor_operations/tensor_algebra.jl:367 [inlined]

[11] #_directsum#251

@ ~/.julia/packages/ITensors/tVfAa/src/tensor_operations/tensor_algebra.jl:375 [inlined]

[12] _directsum

@ ~/.julia/packages/ITensors/tVfAa/src/tensor_operations/tensor_algebra.jl:374 [inlined]

[13] #directsum#259

@ ~/.julia/packages/ITensors/tVfAa/src/tensor_operations/tensor_algebra.jl:495 [inlined]

[14] directsum

@ ~/.julia/packages/ITensors/tVfAa/src/tensor_operations/tensor_algebra.jl:492 [inlined]

[15] #directsum#258

@ ~/.julia/packages/ITensors/tVfAa/src/tensor_operations/tensor_algebra.jl:489 [inlined]

[16] directsum(A_and_I::Pair{ITensor, Index{Int64}}, B_and_J::Pair{ITensor, Index{Int64}})

@ ITensors ~/.julia/packages/ITensors/tVfAa/src/tensor_operations/tensor_algebra.jl:488

[17] macro expansion

@ show.jl:1181 [inlined]

[18] top-level scope

@ ~/Library/CloudStorage/Dropbox/Programs/TENSOR_NETWORK/ITENSOR/FTN/test_directsum.ipynb:1