I think pluggable type systems will become more common in the following years (see also the optional annotations of Python3 that are designed for that too). This is more or less related:
http://bartoszmilewski.wordpress.com/2009/01/18/java-pluggable-types/ (but nonnullability is so basic that it's better inside the language, and not left out to a plug-in type system). Bye, bearophile
