Method selectors can be any object.  In general they will be interned
strings.

Method selectors can be public or private.

Public method selectors are part of a public interface, and are stored
in a loader accessable table by name.  Effectively, anyone who can
name the method selector can get a reference to it.  

Private method selectors are not interned in the global table.  They are
confined by capability semantics, and thus the methods they select are
also so confined.

This should be equivalent to sealers in Joule.

Well, no it's not equivalent.  It doesn't give you a distinction
between the sealer and the unsealer, but it still looks interesting.