+ <news class="beta" date="2019 April 16.">
+ Extended (λδ-2) and restricted (λδ-1) validity is decidable
+ (anniversary milestone).
+ </news>
+ <news class="beta" date="2019 March 25.">
+ Preservation of validity for rt-computation
+ does not need the sort degree parameter
+ (i.e. no induction on the degree).
+ </news>