I have a boolean key 

\keys_define:nn { chessfss }
 { 
   sym .bool_set:N = \l_chessfss_sym_bool 
 }


Now I want to define a "mirrored" key  

no-sym 

so that "no-sym = true" sets  \l_chessfss_sym_bool to false. 

Is there some easy way? 


-- 
Ulrike Fischer