update braiding check

Author: lkdvos

src/tensors/braidingtensor.jl

@@ -19,14 +19,15 @@ struct BraidingTensor{T, S, A <: DenseVector{T}} <: AbstractTensorMap{T, S, 2, 2
     V2::S
     adjoint::Bool
     function BraidingTensor{T, S, A}(V1::S, V2::S, adjoint::Bool = false) where {T, S <: IndexSpace, A <: DenseVector{T}}
-        for a in sectors(V1), b in sectors(V2), c in (a ⊗ b)
-            Nsymbol(a, b, c) == Nsymbol(b, a, c) ||
-                throw(ArgumentError("Cannot define a braiding between $a and $b"))
-        end
+        check_valid_braiding(V1, V1) ||
+            throw(ArgumentError(lazy"Cannot define a braiding between $V1 and $V2"))
         return new{T, S, A}(V1, V2, adjoint)
-        # partial construction: only construct rowr and colr when needed
     end
 end
+
+check_valid_braiding(V₁::S, V₂::S) where {S <: ElementarySpace} =
+    BraidingStyle(sectortype(S)) isa HasBraiding || (isunitspace(V₁) || isunitspace(V₂))
+
 function BraidingTensor{T}(V1::S, V2::S, adjoint::Bool = false) where {T, S <: IndexSpace}
     TA = similarstoragetype(T)
     return BraidingTensor{T, S, TA}(V1, V2, adjoint)