add docstring for fusiontensor

Author: borisdevos

src/irreps/irreps.jl

@@ -59,6 +59,14 @@ Asymbol(a::I, b::I, c::I) where {I <: AbelianIrrep} = Int(Nsymbol(a, b, c))
 Bsymbol(a::I, b::I, c::I) where {I <: AbelianIrrep} = Int(Nsymbol(a, b, c))
 Rsymbol(a::I, b::I, c::I) where {I <: AbelianIrrep} = Int(Nsymbol(a, b, c))
 
+"""
+    fusiontensor(a::I, b::I, c::I) where {I <: AbstractIrrep} -> Array{T, 4}
+
+Return the fusion tensor for the fusion `a ⊗ b -> c`, where `a`, `b`, and `c` are irreps of
+a group. The dimensions of the returned array are `(dim(a), dim(b), dim(c), Nsymbol(a, b, c))`.
+The components of the fusion tensor are simply the Clebsch-Gordan coefficients of the group, 
+describing the unitary basis change from the tensor product of irreps `a` and `b` to the coupled irrep `c`.
+"""
 function fusiontensor(a::I, b::I, c::I) where {I <: AbelianIrrep}
     return fill(Int(Nsymbol(a, b, c)), (1, 1, 1, 1))
 end