julia> using CategoricalArrays, OneHotArrays

julia> cv = CategoricalArrays.CategoricalValue('b', CategoricalArray('a':'z'))
CategoricalValue{Char, UInt32} 'b'

julia> dump(cv)
CategoricalValue{Char, UInt32}
  pool: CategoricalPool{Char, UInt32, CategoricalValue{Char, UInt32}}
    levels: Array{Char}((26,))
      1: Char 'a'
      2: Char 'b'
      3: Char 'c'
      4: Char 'd'
      5: Char 'e'
      ...
      22: Char 'v'
      23: Char 'w'
      24: Char 'x'
      25: Char 'y'
      26: Char 'z'
    invindex: Dict{Char, UInt32}
      slots: Memory{UInt8}
        length: Int64 64
        ptr: Ptr{Nothing} @0x0000000160607020
    ...

julia> cv.pool.levels
26-element Vector{Char}:
 'a': ASCII/Unicode U+0061 (category Ll: Letter, lowercase)
 'b': ASCII/Unicode U+0062 (category Ll: Letter, lowercase)
 'c': ASCII/Unicode U+0063 (category Ll: Letter, lowercase)
 'd': ASCII/Unicode U+0064 (category Ll: Letter, lowercase)
...

julia> Int(cv.ref), length(cv.pool.levels)
(2, 26)

julia> OneHotArrays.onehot(cv::CategoricalValue) = OneHotVector(cv.ref, length(cv.pool.levels))

julia> onehot(cv)
26-element OneHotVector(::UInt32) with eltype Bool:
 ⋅
 1
 ⋅
 ⋅
 ⋅
 ⋅
...

julia> dump(onehot(cv))
OneHotVector{UInt32}
  indices: UInt32 0x00000002
  nlabels: Int64 26