question on aggregate

an example available on the net goes like

df

identifier quantity
1          1       10
2          1       20
3          2       30
4          1       15
5          2       10
6          3       20

aggregate(df$quantity, by=list(df$identifier), sum)

Group.1  x
1       1 45
2       2 40
3       3 20


I'd like Group.1 to retain the name "identifier" and would like to
control what "x" get called in the output.  Thanks.