unexpected sort order with merge

That is odd, I noticed some weird sorting with merge() a while back too and
always am careful with it now.  Fortunately, sort=FALSE seems to work the
way one would think most of the time.  

Although, the following results seem weird too!  (adding by="date" makes it
not sort oddly, regardless of sort=TRUE or FALSE)

merge(d1, d2, by="date", sort=F)

date icpn.x foo icpn.y bar
1 Jan 2000    500   1    500  10
2 Feb 2000    500   2    500  20
3 Mar 2000    500   3    500  30
4 Apr 2000    500   4    500  40
5 May 2000    500   5    500  50
6 Jun 2000    500   6    500  60

merge(d1, d2, by="date", sort=T)

date icpn.x foo icpn.y bar
1 Jan 2000    500   1    500  10
2 Feb 2000    500   2    500  20
3 Mar 2000    500   3    500  30
4 Apr 2000    500   4    500  40
5 May 2000    500   5    500  50
6 Jun 2000    500   6    500  60

merge(d1, d2, by="date")

date icpn.x foo icpn.y bar
1 Jan 2000    500   1    500  10
2 Feb 2000    500   2    500  20
3 Mar 2000    500   3    500  30
4 Apr 2000    500   4    500  40
5 May 2000    500   5    500  50
6 Jun 2000    500   6    500  60

merge(d1, d2)

date icpn foo bar
1 Apr 2000  500   4  40
2 Feb 2000  500   2  20
3 Jan 2000  500   1  10
4 Jun 2000  500   6  60
5 Mar 2000  500   3  30
6 May 2000  500   5  50



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