plyr workaround to converting by() to a data frame

Dear all
Is there a clean plyr version of the following by() and do.call(rbind,
...) construct:

df<-data.frame(a=1:10,b=11:20,c=21:30,grp1=c("x","y"),grp2=c("x","y"),grp3=c("x","y"))
dfsum<-by(df[c("a","b","c")], df[c("grp1","grp2","grp3")], range)
as.data.frame(dfsum)

Error in as.data.frame.default(dfsum) :
  cannot coerce class '"by"' into a data.frame

do.call(rbind, dfsum)

[,1] [,2]
[1,]    1   29
[2,]    2   30


Thank you
Liviu

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