Version 0 of self-modifying code

Self-modifying code is an unusual feature in programming languages, and not one I've seen so beautifully used anywhere else than in Tcl. There are several examples of self modifying procedures on this wiki:

• Let unknown know re-writes unknown
• braintwisters has several examples
• RS's every which "replaces itself" is arguably a case of the same thing
• .. there are plenty more

Procedures that modify themselves in Tcl generally follow a pattern like this:

proc hibye {} {
  puts "Hello, world!"
  proc hibye {} {
    puts "Goodbye, world"
    rename repeat {}
  }
}

 % hibye
 Hello, world!
 % hibye
 Goodbye, world
 % hibye
 invalid command name "hibye"

It is easy to end up in Quoting hell when rewriting procedures.