julia> x = onehotbatch(1:3, 0:4)
5×3 OneHotMatrix(::Vector{UInt32}) with eltype Bool:
 ⋅  ⋅  ⋅
 1  ⋅  ⋅
 ⋅  1  ⋅
 ⋅  ⋅  1
 ⋅  ⋅  ⋅

julia> x[1] = 1;  # really this changes two values, OK?

julia> x[1,3] = 0;  # no effect

julia> x
5×3 OneHotMatrix(::Vector{UInt32}) with eltype Bool:
 1  ⋅  ⋅
 ⋅  ⋅  ⋅
 ⋅  1  ⋅
 ⋅  ⋅  1
 ⋅  ⋅  ⋅

julia> copyto!(x, onehotbatch([4,3,2], 0:4));  # calls setindex! 15 times

julia> x
5×3 OneHotMatrix(::Vector{UInt32}) with eltype Bool:
 ⋅  ⋅  ⋅
 ⋅  ⋅  ⋅
 ⋅  ⋅  1
 ⋅  1  ⋅
 1  ⋅  ⋅

julia> x .= 0  # this is bad, illegal state?
5×3 OneHotMatrix(::Vector{UInt32}) with eltype Bool:
 ⋅  ⋅  ⋅
 ⋅  ⋅  ⋅
 ⋅  ⋅  ⋅
 ⋅  ⋅  ⋅
 ⋅  ⋅  ⋅

julia> onehotbatch(1:10, 0:4, 0)  # with default
5×10 OneHotMatrix(::Vector{UInt32}) with eltype Bool:
 ⋅  ⋅  ⋅  ⋅  1  1  1  1  1  1
 1  ⋅  ⋅  ⋅  ⋅  ⋅  ⋅  ⋅  ⋅  ⋅
 ⋅  1  ⋅  ⋅  ⋅  ⋅  ⋅  ⋅  ⋅  ⋅
 ⋅  ⋅  1  ⋅  ⋅  ⋅  ⋅  ⋅  ⋅  ⋅
 ⋅  ⋅  ⋅  1  ⋅  ⋅  ⋅  ⋅  ⋅  ⋅

julia> onehotbatch(1:10, 0:4)  # without default
ERROR: Value 10 not found in labels

julia> x = onehotbatch(1:3, 0:4);

julia> x[:,3] = collect(onehot(0, 0:4))  # setindex! with array eventually calls this PR's scalar setindex! 5 times
5-element Vector{Bool}:
 1
 0
 0
 0
 0

julia> x
5×3 OneHotMatrix(::Vector{UInt32}) with eltype Bool:
 ⋅  ⋅  1
 1  ⋅  ⋅
 ⋅  1  ⋅
 ⋅  ⋅  ⋅
 ⋅  ⋅  ⋅