[datatable-help] Factors may lose ordered class when used as keys

[Allan Engelhardt]

Is it documented anywhere that factors may lose their ordered-ness when 
used as keys?  E.g.

library("data.table")
F <- factor(LETTERS[1:3], levels = rev(LETTERS), ordered = TRUE)
X<- data.table(A = F, B = F, key = "A")
str(X)                     # A is no longer ordered; B still is
stopifnot(is.ordered(X$B)) # OK
stopifnot(is.ordered(X$A)) # Fails!

I can kind of see why it might happen, but it still caught me by 
surprise, and if it ever happens on (some?) ad-hoc index lookups then it 
will really cause bugs in my code....

Allan
(Above is simpler version of example sent off-list to Matthew)

 > sessionInfo()
R version 2.13.0 (2011-04-13)
Platform: x86_64-unknown-linux-gnu (64-bit)

locale:
  [1] LC_CTYPE=en_GB.utf8       LC_NUMERIC=C
  [3] LC_TIME=en_GB.utf8        LC_COLLATE=en_GB.utf8
  [5] LC_MONETARY=C             LC_MESSAGES=en_GB.utf8
  [7] LC_PAPER=en_GB.utf8       LC_NAME=C
  [9] LC_ADDRESS=C              LC_TELEPHONE=C
[11] LC_MEASUREMENT=en_GB.utf8 LC_IDENTIFICATION=C

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

other attached packages:
[1] data.table_1.6 ctv_0.7-2

loaded via a namespace (and not attached):
[1] tools_2.13.0