let rec weed html aheight = match html with
  | Ast.Words ws -> ()
  | Ast.A(link,body) ->
    if aheight > 0
    then
      raise (Failure "Nested anchors")
    else
      weed body (aheight+1)
  | Ast.B body ->
    weed body aheight
  | Ast.I body ->
    weed body aheight

let weed_html html =
  weed html 0