updating observations in lm

Gentlemans as 274 algorithm allows weights, so adding an obs with a weight of -1 would do the trick of removing obs, too.

This may be a good job for hadwell wickhams c code interface.

Searching for "Gentlemans as 274 algorithm" with google turned up this:

http://jblevins.org/mirror/amiller/

where there is a fortran code for am updated AS 274 algorithm.
I can't judge whether this is suitable for deleting observations.

Berend