Here I discuss why idiom such as S[1,,p,1:3] is problematic (note the
adjacent commas).

First, edit spray.R and change spray_missing_accessor():


`spray_missing_accessor` <- function(S,dots){
  browser()
}	

Then change the first line of the body of `[.spray`() to something like

`[.spray` <- function(S, ...,drop=FALSE){
   spray_missing_accessor(S,dots=match.call())
}	

Build the package with R CMD build --no-build-vignettes

And then install the package.  Fire up R and type:

   library("spray")
   p <- 1:3
   S <- spray(diag(1:6)) ;
   S[1,,p,1:3]


On typing this, you enter the browser; `dots' is a 6-element list.
The first, dots[[1]], is the function name `[.spray` and the second,
dots[[2]], is just S.  This is the *symbol* S, not the variable.  You
can evaluate the symbol by typing eval(dots[[2]]) which shows the
value of S.

The third element, dots[[3]], is just a numeric "1".  The next
element, dots[[4]], corresponding to the gap between the two
consecutive commas, is some sort of ghostly not NULL, not a list,
nothing that I can make sense of.  For example, 'dput(dots[[4]])'
gives an empty line; 'dput(dots[4])' gives "()" ... previous versions
of R gave things like structure(list(), .Names = ""); but note that
structure(list(), .Names = "") will not parse on the commandline.

So, at least prior to R-3.0.0, one cannot use constructions such as
'identical(dots[2], structure(list(), .Names = ""))'.

Now dots[[5]] and indeed dput(dots[[5]]) both return the string `p`.
This can be evaluated with something like eval(dots[[5]]) but you need
to be careful about the environment in which p is to be evaluated.
This is why Jonty had to resort to peculiar tricks with eval.parent()
in Oarray.

Also note that dots[[4]] and dput(dots[[4]]) show "1:3"; note that
class(dots[[6]])is "call", not "numeric" as one might expect.

... quite apart from all this, there is another reason why I have not
implemented things like S[1,,1:3].  Such constructions simply do not
fit nicely into the STL map methods in spray_ops.cpp.  At least I
can't see any nice way.  You'd need some sort of partial matching
system.

I also need to fix Oarray.

