WARNING: This is a warning.
>>>
Document
  { docMeta =
      Meta
        { docTitle = []
        , docTitleAttributes = Nothing
        , docAuthors = []
        , docRevision = Nothing
        , docAttributes = fromList [ ( "sectids" , "" ) ]
        }
  , docBlocks =
      [ Block
          mempty
          Nothing
          (Admonition
             Warning
             [ Block
                 mempty
                 Nothing
                 (Paragraph [ Inline mempty (Str "This is a warning.") ])
             ])
      ]
  }