let get_unique_field ty = match unrollType ty with
| TComp(compinfo,_,_) ->
begin match compinfo.cfields with
| [content_fi] -> content_fi
| _ ->
Jessie_options.fatal "Type %a has no unique field" !Ast_printer.d_type ty
end
| _ -> Jessie_options.fatal "non-struct (unique field)"