Hello! The 'non-sealed' keyword looks not very java-ish. It adds cognitive load, especially to non-native speakers, as it includes the negation inside. Also, it complicates tooling implementation (in particular, lexing Java programs). Probably some of us want to move forward the hyphenated keywords proposal and need a guinea pig to experiment with, but this doesn't seem enough reason to justify its use.
I suggest using the 'open' contextual keyword instead. The 'open' class/interface means that it can be inherited without any restriction. 'open', 'sealed', and 'final' keywords are mutually exclusive. Any class/interface is either 'open', or 'sealed', or 'final', no other option. The classes that don't directly extend/implement sealed classes/interfaces are implicitly 'open' (though it's possible to redundantly specify the 'open' keyword). The classes that extend/implement sealed class/interface must specify open/sealed/final explicitly. What do you think? With best regards, Tagir Valeev.
