[Rd] update fails after specific sequence of steps (PR#474)

[Peter Dalgaard BSA]

Strange things happen on Unix too. 

> source('fox.R')
> Moore<-read.table('Moore.txt')
> attach(Moore)
> mod.full<-lm(conform~status*fcategory)
> summary(mod.full)
...
> mod.m<-update(mod.full, ~.~-status:fcategory)
Error in model.frame.default(formula = ~conform ~ 1, drop.unused.levels = TRUE) : 
	object is not a matrix
> mod.m<-update(mod.full, .~.-status:fcategory) # works following summary and error
> mod.m

Call:
lm(formula = conform ~ status + fcategory)

Coefficients:
    (Intercept)        statuslow     fcategorylow  fcategorymedium  
       14.72356         -4.60667          0.08089         -1.09511  


#### Well you *thought* it worked, but watch: ####
> mod.full<-lm(conform~status*fcategory)
> summary(mod.full)

Call:
lm(formula = conform ~ L׀@ * fcategory)
                       ^^^^^^^

...and the attached dataframe is corrupted. Must investigate further.

> ls("Moore")
[1] "conform"   "fcategory" "fscore"    "L׀@"      "subject"  
                                         ^^^^^^^
-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)             FAX: (+45) 35327907
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