[R] test for nested factors

[Tim Bergsma]

Is there a conventional way to test for nested factors?  I.e., if 'a' 
and 'b' are lists of same-length factors, does each level specified by 
'a' correspond to exactly one level specified by 'b'?

The function below seems to suffice, but I'd be happy to know of a more 
succinct solution, if it already exists.

Thanks,

Tim.

---

"%nested.in%" <- function(x,f,...){
	#coerce to list
	if(!is.list(x))x<-list(x)
	if(!is.list(f))f<-list(f)
	#collapse to vector
	x <- tapply(x[[1]],x)
	f <- tapply(f[[1]],f)
	#analyse
	return(all(sapply(lapply(split(f,x),unique),length)==1))
}

CO2$Plant %nested.in% CO2[,c("Type","Treatment")] #TRUE
CO2$Plant %nested.in% (CO2$uptake < mean(CO2$uptake)) #FALSE