sig
  type heading = [ `H1 | `H2 | `H3 | `H4 | `H5 | `H6 ]
  type block = [ `Address | `Blockquote | `Div | `P | `Pre ]
  type inline =
      [ `Abbr
      | `Acronym
      | `Br
      | `Cite
      | `Code
      | `Dfn
      | `Em
      | `Iframe
      | `Kbd
      | `Q
      | `Samp
      | `Span
      | `Strong
      | `Var ]
  type flow =
      [ `Abbr
      | `Acronym
      | `Address
      | `Blockquote
      | `Br
      | `Cite
      | `Code
      | `Dfn
      | `Div
      | `Em
      | `H1
      | `H2
      | `H3
      | `H4
      | `H5
      | `H6
      | `Iframe
      | `Kbd
      | `P
      | `Pre
      | `Q
      | `Samp
      | `Span
      | `Strong
      | `Var ]
end