anchor.diff:  %% hasobjectpredicate=predicate
   %% Give all things which have any outgoing edge with given predicate
   map_search_field({hasobjectpredicate, Predicate}, _Context) ->
  -    {["o_", z_convert:to_list(Predicate), ":[* TO *]"], []};
  +    {["+o_", z_convert:to_list(Predicate), ":[* TO *]"], []};
   
   %% hassubjectpredicate=predicate
   %% Give all things which have any incoming edge with given predicate
   map_search_field({hassubjectpredicate, Predicate}, _Context) ->
  -    {["s_", z_convert:to_list(Predicate), ":[* TO *]"], []};
  +    {["+s_", z_convert:to_list(Predicate), ":[* TO *]"], []};
   
   %% is_featured or is_featured={false,true}
   %% Filter on whether an item is featured or not.
   map_search_field({is_featured, Bool}, _Context) ->
  -    {["is_featured:", z_convert:to_bool(Bool)],
  +    {["+is_featured:", z_convert:to_bool(Bool)],
        []};
   
   %% upcoming
   %% Filter on items whose end date lies in the future
   map_search_field({upcoming, Boolean}, _Context) ->
       case z_convert:to_bool(Boolean) of
  -        true -> {["date_start:[",
  +        true -> {["+date_start:[",
                     z_convert:to_isotime(erlang:localtime()), " TO *]"],
                    []};
           false -> {[], []}
  
positive.code: map_search_field({hasobject, Id}, _Context) when is_integer(Id) ->
      map_search_field({hasobject, [Id]}, _Context);
  map_search_field({hasobject, [Id]}, _Context) ->
      {["o:", z_convert:to_list(Id)],
       []};
  
  %% hasobject=[id,predicate]
  %% Give all things which have an outgoing edge to Id with the given predicate
  map_search_field({hasobject, [Id, Predicate]}, _Context) ->
      {["o_", z_convert:to_list(Predicate), ":", z_convert:to_list(Id)],
       []};
  map_search_field({hasobject, L}, _Context) when is_list(L) ->
      map_search_field({hasobject, [z_convert:to_integer(L)]}, _Context);
  
negative.code: %%
  
  %% solrfield=field:value
  map_search_field({solr_field, [_, Value]}, _Context)
    when Value =:= undefined orelse Value =:= [] ->
      {[], []};
  map_search_field({solr_field, [Field, Value]}, _Context) ->
      {["+", z_convert:to_list(Field), ":", z_convert:to_list(Value)],
       []};
  
  %% highlight=<field>
  