Expected behaviour of is.unsorted?

Hi,

I've read ?is.unsorted and searched. Have found a few items but nothing
close, yet. Is the following expected?

is.unsorted(data.frame(1:2))
[1] FALSE
is.unsorted(data.frame(2:1))
[1] FALSE
is.unsorted(data.frame(1:2,3:4))
[1] TRUE
is.unsorted(data.frame(2:1,4:3))
[1] TRUE

IIUC, is.unsorted is intended for atomic vectors only (description of x in
?is.unsorted). Indeed the C source (src/main/sort.c) contains an error
message "only atomic vectors can be tested to be sorted". So that is the
error message I expected to see in all cases above, since I know that
data.frame is not an atomic vector. But there is also this in
?is.unsorted: "except for atomic vectors and objects with a class (where
the>= or>  method is used)" which I don't understand. Where>= or>  is
used by what, and where?
If you look at the source, you will see that the basic test for classed 
objects is

all(x[-1L] >= x[-length(x)])

(in the function base:::.gtn).

This comparison doesn't really makes sense for dataframes, but it does 
seem to be backwards:  that tests that x[2] >= x[1], x[3] >= x[2], etc., 
returning TRUE if all comparisons are TRUE:  but that sounds like it 
should be is.sorted(), not is.unsorted().  Or is it my brain that is 
backwards?

Duncan Murdoch
I understand why the first two are FALSE (1 item of anything must be
sorted). I don't understand the 3rd and 4th cases where length is 2:
do_isunsorted seems to call lang3(install(".gtn"), x, CADR(args))). Does
that fall back to TRUE for some reason?

Matthew

sessionInfo()
R version 2.15.0 (2012-03-30)
Platform: x86_64-pc-mingw32/x64 (64-bit)

locale:
[1] LC_COLLATE=English_United Kingdom.1252  LC_CTYPE=English_United
Kingdom.1252
[3] LC_MONETARY=English_United Kingdom.1252 LC_NUMERIC=C
[5] LC_TIME=English_United Kingdom.1252

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

other attached packages:
[1] data.table_1.8.0

loaded via a namespace (and not attached):
[1] tools_2.15.0

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Expected behaviour of is.unsorted?

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