phase separation -- was Re: set-pair!

[JonWilson]

Hi Clinton,

Clinton Ebadi wrote:
> runtime modification of
> lexical environments is incompatible with a properly phase-separated
> compiler).

I hear a lot of mention of phase separation, usually as something
desirable.  However, I've never seen a thorough explanation of why phase
separation is particularly good thing.  (Nor a thorough explanation of
exactly what it is, and google hasn't turned up anything useful so far.)
 Would you mind giving me some pointers to reading material (or, if
you've got nothing which is actually important to do, writing up an
explanation yourself)?
Regards,
Jon