From fe509f797b6e9a20d8026494a305d68cb7bcef5d Mon Sep 17 00:00:00 2001
From: lkdvos <ldevos98@gmail.com>
Date: Tue, 18 Aug 2026 10:09:26 +0200
Subject: [PATCH 1/2] overload BraidingTensor constructors properly

---
 src/tensors/tensoroperations.jl | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/src/tensors/tensoroperations.jl b/src/tensors/tensoroperations.jl
index a6f660b..a4fb563 100644
--- a/src/tensors/tensoroperations.jl
+++ b/src/tensors/tensoroperations.jl
@@ -119,6 +119,12 @@ function TK.BraidingTensor{T, S}(
         V1::SumSpace{S}, V2::SumSpace{S}, adjoint::Bool = false
     ) where {T, S}
     τtype = TK.braidingtensortype(S, T)
+    return τtype(V1, V2, adjoint)
+end
+function TK.BraidingTensor{T, S, A}(
+        V1::SumSpace{S}, V2::SumSpace{S}, adjoint::Bool = false
+    ) where {T, S, A}
+    τtype = BraidingTensor{T, S, A}
     cod, dom = adjoint ? (V1 ⊗ V2, V2 ⊗ V1) : (V2 ⊗ V1, V1 ⊗ V2)
     tdst = SparseBlockTensorMap{τtype}(undef, cod, dom)
     Vs = eachspace(tdst)
@@ -130,3 +136,12 @@ function TK.BraidingTensor{T, S}(
     end
     return tdst
 end
+
+TK.braidingtensortype(::Type{SumSpace{S}}, ::Type{TorA}) where {S <: IndexSpace, TorA} =
+    TK.braidingtensortype(S, TorA)
+
+# TODO: remove once proper promotion rules in TensorKit are in place
+TK.BraidingTensor{T, S, A}(V1::S, V2::SumSpace{S}, adjoint::Bool = false) where {T, S, A} =
+    BraidingTensor{T, S, A}(promote(V1, V2)..., adjoint)
+TK.BraidingTensor{T, S, A}(V1::SumSpace{S}, V2::S, adjoint::Bool = false) where {T, S, A} =
+    BraidingTensor{T, S, A}(promote(V1, V2)..., adjoint)

From 7087c48eac435ed2fcf9e84a0f5179a281715293 Mon Sep 17 00:00:00 2001
From: lkdvos <ldevos98@gmail.com>
Date: Tue, 18 Aug 2026 10:09:32 +0200
Subject: [PATCH 2/2] add regression test

---
 test/abstracttensor/braidingtensor.jl | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/test/abstracttensor/braidingtensor.jl b/test/abstracttensor/braidingtensor.jl
index bb0a897..14ac8c8 100644
--- a/test/abstracttensor/braidingtensor.jl
+++ b/test/abstracttensor/braidingtensor.jl
@@ -1,6 +1,7 @@
 using Test
 using TestExtras
 using TensorKit
+using TensorKit: ℙ
 using BlockTensorKit
 using Random
 
@@ -34,3 +35,14 @@ Random.seed!(1234)
         @test !(y ≈ braid(x, ((2, 1), (3,)), (2, 1, 3)))
     end
 end
+
+@testset "Issue #68" begin
+    x = randn(ComplexF64, (⊞(ℙ^1) ⊗ ⊞(ℙ^1)) ← ⊞(ℙ^1))
+    O = randn(ComplexF64, ((ℙ^1 ⊞ ℙ^4 ⊞ ℙ^16 ⊞ ℙ^4) ⊗ ⊞(ℙ^2)) ← (⊞(ℙ^2) ⊗ ⊞(ℙ^1)))
+    A = randn(ComplexF64, (ℙ^4 ⊗ F^2 ⊗ (ℙ^2)') ← ℙ^1)
+    Ab = randn(ComplexF64, (ℙ^4 ⊗ ℙ^2 ⊗ (ℙ^2)') ← ℙ^1)
+
+    @plansor y[-1 -2; -3] ≔ A[-1 4 2; 1] * O[-2 6; 4 5] * τ[5 7; 2 3] *
+        conj(Ab[-3 6 7; 8]) * x[1 3; 8]
+    @test space(y) == (⊞(ℙ^4) ⊗ ⊞(ℙ^1)) ← ⊞(ℙ^4)
+end