Remember what I said about Smalltalk being flexible because it's implemented in itself?

Here's Avi, the author of Seaside :

Ironically, in that moment [realizing he could use continuations in the server] I was at first convinced that I would have to move my code back to Ruby, because Smalltalk doesn?t have primitive support for capturing continuations as Ruby does. The subsequent experience of realizing that they could be implemented in Smalltalk at the library level, simply by some trivial manipulations of the stack frame objects, was one of those that really cemented my choice of environment.

A bit of history on Seaside.