If needed, I'll update the PEP to be more clear.  The sets.py module 
had two protocols: __as_immutable__() and __as_temporarily_immutable__
().  The first protocol was the one that was dropped -- it triggered 
with something like "s1.add(s2)" and it automatically created a 
frozenset copy of s2.  The second protocol is used in 
contains/remove/discard.  Alex Martelli successfully lobbied for its 
retention.