Awesome. Mantra does a dynamic mixin. Just map a string to a closure with "self" as first arg:

int.mixin("toHex",
   {int self | return java {new mstring(Integer.toHexString(((mint)self).v))};}
);
println(32.toHex()); // emits 20, the hex verison of 32. :)

also note you could alter the MetaClass per object (set .class field) to alter behavior per instance! You can ask about meta object now:

println(32.class); // emits <int>

To avoid strange bugs, you can only add not override/alter behavior.