unexpected '[<-.data.frame' result

Is this a bug?

If not, I am curious to know why '[<-.data.frame' was designed to yield 
a.frame$y != a.frame$z rather than refusing to carry out the operation at 
all.

a.frame <- data.frame( x=letters[1:5] )
a.frame[ 2:5, "y" ] <- letters[2:5]
a.frame[[ "z" ]][ 2:5 ] <- letters[2:5]
a.frame

x    y    z
1 a    b <NA>
2 b    c    b
3 c    d    c
4 d    e    d
5 e <NA>    e

Chuck


Charles C. Berry                        (858) 534-2098
                                          Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	         UC San Diego
http://biostat.ucsd.edu/~cberry/         La Jolla, San Diego 92093-0717