! org-mode compatibility
   ! element order
      ! SCHEDULED:
      ! DEADLINE:
      ! :properties:
        > aka drawers
      ! ctl lines
         > create, modify, finish dates
      ! > callout lines
      ! ! <link>
      ! any other text lines

   ! app:org limitations
      ! no nested drawers

   ! app:org extensions
      ! arbitrary element orders
         > fixed to org-mode order when writing .org files
      ! todo.txt style trees
         ! including properties
      ! ctl lines
         ! start with "! "
         ! create, modify, finish dates
            ! "! 11Jan2020" (create)
            ! "! C:12Jan2020" (modify/change)
               ! may appear multiple times
            ! "! F:13Jan2020" (finish)
         ! link
      ! callouts
         ! text callout via "> my callout text line"
            > may appear multiple times
         ! image callout via "> images/myimage.png"
            > may only appear once
      ! info nodes before first task node
         > e.g. for research notes
      ! support for taskjuggler properties
         ! effort
         ! resource_id
         ! allocated
         ! blocker
         ! depends
         ! limits
            > workhours
         ! extensions
            ! resource_type
               > dev,group,pm, ..
            ! vacation
      ! preserves mindmap UI settings via "smmx-" properties
         ! positions, text styles, shape styles, font sizes, colors, ..
