setdiff bizarre (was: odd behavior out of setdiff)

but simply treats the data frame as a *character* list:

? ? 1 %in% data.frame(a=2,b=1) ?# TRUE
? ? '1' %in% data.frame(a=2,b=1) ?# TRUE
? ? 1 %in% data.frame(a=2:3,b=1:2) # FALSE
? ? 1:3 %in% data.frame(a=2:4,b=1:3) ?# FALSE FALSE FALSE
? ? '1:3' %in% data.frame(a=2:4,b=1:3) ?# TRUE

It applies as.character to the dataframe:

 > z=data.frame(a=2:4,b=1:3)
 > as.character(z)
 [1] "2:4" "1:3"

  The as.character method for data frames seems to spot integer
sequences (but only for int types and not num types) and show the a:b
notation:

 > x=data.frame(z=as.integer(c(1,2,3,4,5)))
 > str(x)
 'data.frame':	5 obs. of  1 variable:
  $ z: int  1 2 3 4 5
 > as.character(x)
 [1] "1:5"

 Obviously it doesn't do this for vectors:

 > as.character(x$z)
 [1] "1" "2" "3" "4" "5"

 I suspect it's using 'deparse()' to get the character representation.
This function is mentioned in ?as.character, but as.character.default
disappears into the infernal .Internal and I don't have time to chase
source code - it's sunny outside!

Barry