In message "[ruby-talk:01016] Re: Reparenting a block"
    on 99/12/31, Dave Thomas <Dave / thomases.com> writes:

|In fact my interest started after I started poking around inside Tk
|and came across the technique. However, I then saw that you did the
|instance_eval in a C extension, rather than directly in Ruby, so I got 
|to wondering.

It is because of performace/historical reason.