sec_is_open_to_public()/sec_is_closed_to_public()

dear developers

i'm using get_public_tree() in my configurable table of contents code  
source. but when part of the site is protected by viewer roles i get  
the error

   "You are not allowed to access 'get_viewable' in this context"

looking through the api i found the functions sec_is_open_to_public()  
and sec_is_closed_to_public() which i could use exactly to solve the  
above problem by including them in a tal:condition. but all i get are  
errors. to get more debugging info i hack around in the silva source  
code (https://infrae.com/issue/silva/issue1289) but i'm not getting  
much smarter. how is infrae using these functions? i egrep through  
the complete silva source code and lo and behold these functions are  
nowhere to be found! did i stumble upon some obsolete functions? i'm  
lost here. any advise?

regards,
marc