conservatism in extensions

[Kent M. Pitman <KMP at MIT-MC>]

i agree. we should be very skeptical of extending things when there is wide
variety of opinion on the direction that extension should go. certainly at
this phase, extensions of this class are not advisable at all. we can always
later add such functionality in an upward-compatible way; it will be harder
to go back on wrong decisions. right now, we need to get any hard issues
ironed out that are needed to get initial implementations up and running.
once people have played with implementations for a while and we get back
feedback about actual use, then is the time to extend an operation...