diff --git a/src/QuantumInterface.jl b/src/QuantumInterface.jl
index 34efa04..89d3c9a 100644
--- a/src/QuantumInterface.jl
+++ b/src/QuantumInterface.jl
@@ -1,14 +1,39 @@
 module QuantumInterface
 
-import Base: ==, +, -, *, /, ^, length, one, exp, conj, conj!, transpose, copy
-import LinearAlgebra: tr, ishermitian, norm, normalize, normalize!
-import Base: show, summary
-import SparseArrays: sparse, spzeros, AbstractSparseMatrix # TODO move to an extension
+##
+# Basis specific
+##
+
+"""
+    basis(a)
+
+Return the basis of an object.
+
+If it's ambiguous, e.g. if an operator has a different left and right basis,
+an [`IncompatibleBases`](@ref) error is thrown.
+"""
+function basis end
+
+"""
+Exception that should be raised for an illegal algebraic operation.
+"""
+mutable struct IncompatibleBases <: Exception end
+
+
+##
+# Standard methods
+##
 
 function apply! end
 
 function dagger end
 
+"""
+    directsum(x, y, z...)
+
+Direct sum of the given objects. Alternatively, the unicode
+symbol ⊕ (\\oplus) can be used.
+"""
 function directsum end
 const ⊕ = directsum
 directsum() = GenericBasis(0)
@@ -86,8 +111,9 @@ function squeeze end
 function wigner end
 
 
-include("bases.jl")
 include("abstract_types.jl")
+include("bases.jl")
+include("show.jl")
 
 include("linalg.jl")
 include("tensor.jl")
diff --git a/src/abstract_types.jl b/src/abstract_types.jl
index f8667c9..0650290 100644
--- a/src/abstract_types.jl
+++ b/src/abstract_types.jl
@@ -1,3 +1,18 @@
+"""
+Abstract base class for all specialized bases.
+
+The Basis class is meant to specify a basis of the Hilbert space of the
+studied system. Besides basis specific information all subclasses must
+implement a shape variable which indicates the dimension of the used
+Hilbert space. For a spin-1/2 Hilbert space this would be the
+vector `[2]`. A system composed of two spins would then have a
+shape vector `[2 2]`.
+
+Composite systems can be defined with help of the [`CompositeBasis`](@ref)
+class.
+"""
+abstract type Basis end
+
 """
 Abstract base class for `Bra` and `Ket` states.
 
@@ -38,20 +53,3 @@ A_{br_1,br_2} = B_{bl_1,bl_2} S_{(bl_1,bl_2) ↔ (br_1,br_2)}
 ```
 """
 abstract type AbstractSuperOperator{B1,B2} end
-
-function summary(stream::IO, x::AbstractOperator)
-    print(stream, "$(typeof(x).name.name)(dim=$(length(x.basis_l))x$(length(x.basis_r)))\n")
-    if samebases(x)
-        print(stream, "  basis: ")
-        show(stream, basis(x))
-    else
-        print(stream, "  basis left:  ")
-        show(stream, x.basis_l)
-        print(stream, "\n  basis right: ")
-        show(stream, x.basis_r)
-    end
-end
-
-show(stream::IO, x::AbstractOperator) = summary(stream, x)
-
-traceout!(s::StateVector, i) = ptrace(s,i)
diff --git a/src/bases.jl b/src/bases.jl
index 6e4b077..042f905 100644
--- a/src/bases.jl
+++ b/src/bases.jl
@@ -1,18 +1,3 @@
-"""
-Abstract base class for all specialized bases.
-
-The Basis class is meant to specify a basis of the Hilbert space of the
-studied system. Besides basis specific information all subclasses must
-implement a shape variable which indicates the dimension of the used
-Hilbert space. For a spin-1/2 Hilbert space this would be the
-vector `[2]`. A system composed of two spins would then have a
-shape vector `[2 2]`.
-
-Composite systems can be defined with help of the [`CompositeBasis`](@ref)
-class.
-"""
-abstract type Basis end
-
 """
     length(b::Basis)
 
@@ -20,17 +5,6 @@ Total dimension of the Hilbert space.
 """
 Base.length(b::Basis) = prod(b.shape)
 
-"""
-    basis(a)
-
-Return the basis of an object.
-
-If it's ambiguous, e.g. if an operator has a different left and right basis,
-an [`IncompatibleBases`](@ref) error is thrown.
-"""
-function basis end
-
-
 """
     GenericBasis(N)
 
@@ -137,11 +111,6 @@ function equal_bases(a, b)
     return true
 end
 
-"""
-Exception that should be raised for an illegal algebraic operation.
-"""
-mutable struct IncompatibleBases <: Exception end
-
 const BASES_CHECK = Ref(true)
 
 """
@@ -366,9 +335,9 @@ SumBasis(shape, bases::Vector) = (tmp = (bases...,); SumBasis(shape, tmp))
 SumBasis(bases::Vector) = SumBasis((bases...,))
 SumBasis(bases::Basis...) = SumBasis((bases...,))
 
-==(b1::T, b2::T) where T<:SumBasis = equal_shape(b1.shape, b2.shape)
-==(b1::SumBasis, b2::SumBasis) = false
-length(b::SumBasis) = sum(b.shape)
+Base.:(==)(b1::T, b2::T) where T<:SumBasis = equal_shape(b1.shape, b2.shape)
+Base.:(==)(b1::SumBasis, b2::SumBasis) = false
+Base.length(b::SumBasis) = sum(b.shape)
 
 """
     directsum(b1::Basis, b2::Basis)
@@ -393,62 +362,3 @@ function directsum(b1::SumBasis, b2::SumBasis)
     bases = [b1.bases...;b2.bases...]
     return SumBasis(shape, (bases...,))
 end
-
-embed(b::SumBasis, indices, ops) = embed(b, b, indices, ops)
-
-##
-# show methods
-##
-
-function show(stream::IO, x::GenericBasis)
-    if length(x.shape) == 1
-        write(stream, "Basis(dim=$(x.shape[1]))")
-    else
-        s = replace(string(x.shape), " " => "")
-        write(stream, "Basis(shape=$s)")
-    end
-end
-
-function show(stream::IO, x::CompositeBasis)
-    write(stream, "[")
-    for i in 1:length(x.bases)
-        show(stream, x.bases[i])
-        if i != length(x.bases)
-            write(stream, " ⊗ ")
-        end
-    end
-    write(stream, "]")
-end
-
-function show(stream::IO, x::SpinBasis)
-    d = denominator(x.spinnumber)
-    n = numerator(x.spinnumber)
-    if d == 1
-        write(stream, "Spin($n)")
-    else
-        write(stream, "Spin($n/$d)")
-    end
-end
-
-function show(stream::IO, x::FockBasis)
-    if iszero(x.offset)
-        write(stream, "Fock(cutoff=$(x.N))")
-    else
-        write(stream, "Fock(cutoff=$(x.N), offset=$(x.offset))")
-    end
-end
-
-function show(stream::IO, x::NLevelBasis)
-    write(stream, "NLevel(N=$(x.N))")
-end
-
-function show(stream::IO, x::SumBasis)
-    write(stream, "[")
-    for i in 1:length(x.bases)
-        show(stream, x.bases[i])
-        if i != length(x.bases)
-            write(stream, " ⊕ ")
-        end
-    end
-    write(stream, "]")
-end
diff --git a/src/embed_permute.jl b/src/embed_permute.jl
index c2cc4ca..ac63bc4 100644
--- a/src/embed_permute.jl
+++ b/src/embed_permute.jl
@@ -67,8 +67,8 @@ function embed(basis_l::CompositeBasis, basis_r::CompositeBasis,
     ops_sb = [x[2] for x in idxop_sb]
 
     for (idxsb, opsb) in zip(indices_sb, ops_sb)
-        (opsb.basis_l == basis_l.bases[idxsb]) || throw(IncompatibleBases())
-        (opsb.basis_r == basis_r.bases[idxsb]) || throw(IncompatibleBases())
+        (opsb.basis_l == basis_l.bases[idxsb]) || throw(IncompatibleBases()) # FIXME issue #12
+        (opsb.basis_r == basis_r.bases[idxsb]) || throw(IncompatibleBases()) # FIXME issue #12
     end
 
     S = length(operators) > 0 ? mapreduce(eltype, promote_type, operators) : Any
@@ -83,6 +83,8 @@ function embed(basis_l::CompositeBasis, basis_r::CompositeBasis,
     return embed_op
 end
 
+embed(b::SumBasis, indices, ops) = embed(b, b, indices, ops)
+
 permutesystems(a::AbstractOperator, perm) = arithmetic_unary_error("Permutations of subsystems", a)
 
 nsubsystems(s::AbstractKet) = nsubsystems(basis(s))
diff --git a/src/identityoperator.jl b/src/identityoperator.jl
index 5959882..aa031ff 100644
--- a/src/identityoperator.jl
+++ b/src/identityoperator.jl
@@ -1,4 +1,4 @@
-one(x::Union{<:Basis,<:AbstractOperator}) = identityoperator(x)
+Base.one(x::Union{<:Basis,<:AbstractOperator}) = identityoperator(x)
 
 """
     identityoperator(a::Basis[, b::Basis])
@@ -22,4 +22,4 @@ identityoperator(::Type{T}, ::Type{Any}, b1::Basis, b2::Basis) where T<:Abstract
 identityoperator(b1::Basis, b2::Basis) = identityoperator(ComplexF64, b1, b2)
 
 """Prepare the identity superoperator over a given space."""
-function identitysuperoperator end
\ No newline at end of file
+function identitysuperoperator end
diff --git a/src/julia_base.jl b/src/julia_base.jl
index 9a0532d..126694c 100644
--- a/src/julia_base.jl
+++ b/src/julia_base.jl
@@ -1,3 +1,5 @@
+import Base: +, -, *, /, ^, length, exp, conj, conj!, adjoint, transpose, copy
+
 # Common error messages
 arithmetic_unary_error(funcname, x::AbstractOperator) = throw(ArgumentError("$funcname is not defined for this type of operator: $(typeof(x)).\nTry to convert to another operator type first with e.g. dense() or sparse()."))
 arithmetic_binary_error(funcname, a::AbstractOperator, b::AbstractOperator) = throw(ArgumentError("$funcname is not defined for this combination of types of operators: $(typeof(a)), $(typeof(b)).\nTry to convert to a common operator type first with e.g. dense() or sparse()."))
@@ -8,33 +10,33 @@ addnumbererror() = throw(ArgumentError("Can't add or subtract a number and an op
 # States
 ##
 
--(a::T) where {T<:StateVector} = T(a.basis, -a.data)
+-(a::T) where {T<:StateVector} = T(a.basis, -a.data) # FIXME issue #12
 *(a::StateVector, b::Number) = b*a
-copy(a::T) where {T<:StateVector} = T(a.basis, copy(a.data))
-length(a::StateVector) = length(a.basis)::Int
-basis(a::StateVector) = a.basis
+copy(a::T) where {T<:StateVector} = T(a.basis, copy(a.data)) # FIXME issue #12
+length(a::StateVector) = length(a.basis)::Int # FIXME issue #12
+basis(a::StateVector) = a.basis # FIXME issue #12
 directsum(x::StateVector...) = reduce(directsum, x)
+adjoint(a::StateVector) = dagger(a)
+
+
 
 # Array-like functions
-Base.size(x::StateVector) = size(x.data)
-@inline Base.axes(x::StateVector) = axes(x.data)
+Base.size(x::StateVector) = size(x.data) # FIXME issue #12
+@inline Base.axes(x::StateVector) = axes(x.data) # FIXME issue #12
 Base.ndims(x::StateVector) = 1
 Base.ndims(::Type{<:StateVector}) = 1
-Base.eltype(x::StateVector) = eltype(x.data)
+Base.eltype(x::StateVector) = eltype(x.data) # FIXME issue #12
 
 # Broadcasting
 Base.broadcastable(x::StateVector) = x
 
-Base.adjoint(a::StateVector) = dagger(a)
-
-
 ##
 # Operators
 ##
 
-length(a::AbstractOperator) = length(a.basis_l)::Int*length(a.basis_r)::Int
-basis(a::AbstractOperator) = (check_samebases(a); a.basis_l)
-basis(a::AbstractSuperOperator) = (check_samebases(a); a.basis_l[1])
+length(a::AbstractOperator) = length(a.basis_l)::Int*length(a.basis_r)::Int  # FIXME issue #12
+basis(a::AbstractOperator) = (check_samebases(a); a.basis_l) # FIXME issue #12
+basis(a::AbstractSuperOperator) = (check_samebases(a); a.basis_l[1]) # FIXME issue #12
 
 # Ensure scalar broadcasting
 Base.broadcastable(x::AbstractOperator) = Ref(x)
@@ -60,14 +62,17 @@ Operator exponential.
 """
 exp(op::AbstractOperator) = throw(ArgumentError("exp() is not defined for this type of operator: $(typeof(op)).\nTry to convert to dense operator first with dense()."))
 
-Base.size(op::AbstractOperator) = (length(op.basis_l),length(op.basis_r))
+Base.size(op::AbstractOperator) = (length(op.basis_l),length(op.basis_r)) # FIXME issue #12
 function Base.size(op::AbstractOperator, i::Int)
     i < 1 && throw(ErrorException("dimension index is < 1"))
     i > 2 && return 1
-    i==1 ? length(op.basis_l) : length(op.basis_r)
+    i==1 ? length(op.basis_l) : length(op.basis_r) # FIXME issue #12
 end
 
-Base.adjoint(a::AbstractOperator) = dagger(a)
+adjoint(a::AbstractOperator) = dagger(a)
+
+transpose(a::AbstractOperator) = arithmetic_unary_error("Transpose", a)
+
 
 conj(a::AbstractOperator) = arithmetic_unary_error("Complex conjugate", a)
 conj!(a::AbstractOperator) = conj(a::AbstractOperator)
diff --git a/src/julia_linalg.jl b/src/julia_linalg.jl
index d2f4d3d..3087d0a 100644
--- a/src/julia_linalg.jl
+++ b/src/julia_linalg.jl
@@ -1,3 +1,5 @@
+import LinearAlgebra: tr, ishermitian, norm, normalize, normalize!
+
 """
     ishermitian(op::AbstractOperator)
 
@@ -17,7 +19,7 @@ tr(x::AbstractOperator) = arithmetic_unary_error("Trace", x)
 
 Norm of the given bra or ket state.
 """
-norm(x::StateVector) = norm(x.data)
+norm(x::StateVector) = norm(x.data) # FIXME issue #12
 
 """
     normalize(x::StateVector)
@@ -31,7 +33,7 @@ normalize(x::StateVector) = x/norm(x)
 
 In-place normalization of the given bra or ket so that `norm(x)` is one.
 """
-normalize!(x::StateVector) = (normalize!(x.data); x)
+normalize!(x::StateVector) = (normalize!(x.data); x) # FIXME issue #12
 
 """
     normalize(op)
diff --git a/src/linalg.jl b/src/linalg.jl
index 8bb47cd..fb0779d 100644
--- a/src/linalg.jl
+++ b/src/linalg.jl
@@ -1,10 +1,10 @@
-samebases(a::AbstractOperator) = samebases(a.basis_l, a.basis_r)::Bool
-samebases(a::AbstractOperator, b::AbstractOperator) = samebases(a.basis_l, b.basis_l)::Bool && samebases(a.basis_r, b.basis_r)::Bool
-check_samebases(a::Union{AbstractOperator, AbstractSuperOperator}) = check_samebases(a.basis_l, a.basis_r)
-multiplicable(a::AbstractOperator, b::AbstractOperator) = multiplicable(a.basis_r, b.basis_l)
+samebases(a::AbstractOperator) = samebases(a.basis_l, a.basis_r)::Bool # FIXME issue #12
+samebases(a::AbstractOperator, b::AbstractOperator) = samebases(a.basis_l, b.basis_l)::Bool && samebases(a.basis_r, b.basis_r)::Bool # FIXME issue #12
+check_samebases(a::Union{AbstractOperator, AbstractSuperOperator}) = check_samebases(a.basis_l, a.basis_r) # FIXME issue #12
+multiplicable(a::AbstractOperator, b::AbstractOperator) = multiplicable(a.basis_r, b.basis_l) # FIXME issue #12
 dagger(a::AbstractOperator) = arithmetic_unary_error("Hermitian conjugate", a)
-transpose(a::AbstractOperator) = arithmetic_unary_error("Transpose", a)
 directsum(a::AbstractOperator...) = reduce(directsum, a)
 ptrace(a::AbstractOperator, index) = arithmetic_unary_error("Partial trace", a)
 _index_complement(b::CompositeBasis, indices) = complement(length(b.bases), indices)
 reduced(a, indices) = ptrace(a, _index_complement(basis(a), indices))
+traceout!(s::StateVector, i) = ptrace(s,i)
diff --git a/src/show.jl b/src/show.jl
new file mode 100644
index 0000000..38607b0
--- /dev/null
+++ b/src/show.jl
@@ -0,0 +1,69 @@
+import Base: show, summary
+
+function summary(stream::IO, x::AbstractOperator)
+    print(stream, "$(typeof(x).name.name)(dim=$(length(x.basis_l))x$(length(x.basis_r)))\n")
+    if samebases(x)
+        print(stream, "  basis: ")
+        show(stream, basis(x))
+    else
+        print(stream, "  basis left:  ")
+        show(stream, x.basis_l)
+        print(stream, "\n  basis right: ")
+        show(stream, x.basis_r)
+    end
+end
+
+show(stream::IO, x::AbstractOperator) = summary(stream, x)
+
+function show(stream::IO, x::GenericBasis)
+    if length(x.shape) == 1
+        write(stream, "Basis(dim=$(x.shape[1]))")
+    else
+        s = replace(string(x.shape), " " => "")
+        write(stream, "Basis(shape=$s)")
+    end
+end
+
+function show(stream::IO, x::CompositeBasis)
+    write(stream, "[")
+    for i in 1:length(x.bases)
+        show(stream, x.bases[i])
+        if i != length(x.bases)
+            write(stream, " ⊗ ")
+        end
+    end
+    write(stream, "]")
+end
+
+function show(stream::IO, x::SpinBasis)
+    d = denominator(x.spinnumber)
+    n = numerator(x.spinnumber)
+    if d == 1
+        write(stream, "Spin($n)")
+    else
+        write(stream, "Spin($n/$d)")
+    end
+end
+
+function show(stream::IO, x::FockBasis)
+    if iszero(x.offset)
+        write(stream, "Fock(cutoff=$(x.N))")
+    else
+        write(stream, "Fock(cutoff=$(x.N), offset=$(x.offset))")
+    end
+end
+
+function show(stream::IO, x::NLevelBasis)
+    write(stream, "NLevel(N=$(x.N))")
+end
+
+function show(stream::IO, x::SumBasis)
+    write(stream, "[")
+    for i in 1:length(x.bases)
+        show(stream, x.bases[i])
+        if i != length(x.bases)
+            write(stream, " ⊕ ")
+        end
+    end
+    write(stream, "]")
+end
diff --git a/src/sparse.jl b/src/sparse.jl
index 2ba8f5f..d6b301c 100644
--- a/src/sparse.jl
+++ b/src/sparse.jl
@@ -1,4 +1,4 @@
-# TODO make an extension?
+import SparseArrays: sparse, spzeros, AbstractSparseMatrix # TODO move to an extension
 
 # dense(a::AbstractOperator) = arithmetic_unary_error("Conversion to dense", a)
 
diff --git a/test/runtests.jl b/test/runtests.jl
index 0bccf25..826fe33 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -26,6 +26,7 @@ end
 println("Starting tests with $(Threads.nthreads()) threads out of `Sys.CPU_THREADS = $(Sys.CPU_THREADS)`...")
 
 @doset "sortedindices"
+@doset "bases"
 #VERSION >= v"1.9" && @doset "doctests"
 get(ENV,"JET_TEST","")=="true" && @doset "jet"
 VERSION >= v"1.9" && @doset "aqua"
diff --git a/test/test_bases.jl b/test/test_bases.jl
new file mode 100644
index 0000000..1d91673
--- /dev/null
+++ b/test/test_bases.jl
@@ -0,0 +1,55 @@
+using Test
+using QuantumInterface: tensor, ⊗, ptrace, reduced, permutesystems, equal_bases, multiplicable
+using QuantumInterface: GenericBasis, CompositeBasis, NLevelBasis, FockBasis
+
+@testset "basis" begin
+
+shape1 = [5]
+shape2 = [2, 3]
+shape3 = [6]
+
+b1 = GenericBasis(shape1)
+b2 = GenericBasis(shape2)
+b3 = GenericBasis(shape3)
+
+@test b1.shape == shape1
+@test b2.shape == shape2
+@test b1 != b2
+@test b1 != FockBasis(2)
+@test b1 == b1
+
+@test tensor(b1) == b1
+comp_b1 = tensor(b1, b2)
+comp_uni = b1 ⊗ b2
+comp_b2 = tensor(b1, b1, b2)
+@test comp_b1.shape == [prod(shape1), prod(shape2)]
+@test comp_uni.shape == [prod(shape1), prod(shape2)]
+@test comp_b2.shape == [prod(shape1), prod(shape1), prod(shape2)]
+
+@test b1^3 == CompositeBasis(b1, b1, b1)
+@test (b1⊗b2)^2 == CompositeBasis(b1, b2, b1, b2)
+@test_throws ArgumentError b1^(0)
+
+comp_b1_b2 = tensor(comp_b1, comp_b2)
+@test comp_b1_b2.shape == [prod(shape1), prod(shape2), prod(shape1), prod(shape1), prod(shape2)]
+@test comp_b1_b2 == CompositeBasis(b1, b2, b1, b1, b2)
+
+@test_throws ArgumentError tensor()
+@test comp_b2.shape == tensor(b1, comp_b1).shape
+@test comp_b2 == tensor(b1, comp_b1)
+
+@test_throws ArgumentError ptrace(comp_b1, [1, 2])
+@test ptrace(comp_b2, [1]) == ptrace(comp_b2, [2]) == comp_b1 == ptrace(comp_b2, 1)
+@test ptrace(comp_b2, [1, 2]) == ptrace(comp_b1, [1])
+@test ptrace(comp_b2, [2, 3]) == ptrace(comp_b1, [2])
+@test ptrace(comp_b2, [2, 3]) == reduced(comp_b2, [1])
+@test_throws ArgumentError reduced(comp_b1, [])
+
+comp1 = tensor(b1, b2, b3)
+comp2 = tensor(b2, b1, b3)
+@test permutesystems(comp1, [2,1,3]) == comp2
+
+@test !equal_bases([b1, b2], [b1, b3])
+@test !multiplicable(comp1, b1 ⊗ b2 ⊗ NLevelBasis(prod(b3.shape)))
+
+end # testset