[R] using variables in obj$model

[Vito Muggeo]

dear all,
for some reason I am intersted in updating a glm taking variables from its
model argument, namely:

> dati<-data.frame(y=runif(10),x=1:10)
> obj<-glm(y~x,data=dati)
> obj$model[,c("A","a:b")]<-cbind(rnorm(10),runif(10))
> names(obj$model)
[1] "y"   "x"   "A"   "a:b"
> update(obj,.~.+A,data=obj$model) #it works

Call:  glm(formula = y ~ x + A, data = obj$model)
............[SNIP]

> update(obj,.~.+A+a:b,data=obj$model) #it does not work
Error in eval(expr, envir, enclos) : Object "a" not found


Please, how can I solve this problem?
many thanks in avance,
best,
vito