let rec transform html bdepth = match html with
  | Ast.Words ws -> 
    html
  | Ast.A(link,body) ->
    let body' = transform body bdepth in
    Ast.A(link,body')
  | Ast.B body ->
    let body' = transform body (bdepth+1) in
    if bdepth>0
    then body'
    else Ast.B body'
  | Ast.I body ->
    let body' = transform body bdepth in
    Ast.I body'

let transform_html html =
  transform html 0