anchor.diff:  m_value(_Source, _Context) ->
       undefined.
   
  +%% @doc Get URL to license at the Creative Commons website
  +-spec url_for(binary() | string()) -> binary() | undefined.
   url_for(License) when is_list(License) ->
       url_for(z_convert:to_binary(License));
   url_for(<<"BY">>) ->
  
positive.code:     m_find_value/3,
      m_to_list/2,
      m_value/2,
      url_for/1
  ]).
  
  -include_lib("zotonic.hrl").
  
negative.code:     <<"http://creativecommons.org/publicdomain/mark/1.0">>;
  url_for(_) ->
      undefined.
  
  %% @doc Get URL to translated license at the Creative Commons website
  -spec language_url_for(binary(), #context{}) -> binary() | undefined.
  language_url_for(License, Context) ->
      with_language(url_for(License), Context).
  
  -spec with_language(binary() | undefined, #context{}) -> binary() | undefined.
  with_language(undefined, _Context) ->
      undefined;
  with_language(Url, Context) ->
      iolist_to_binary([
          Url,
          <<"/deed.">>,
          z_convert:to_binary(z_context:language(Context))
      ]).
  