problem with unlist

From: r.ghezzo

Hello, I try to compute means,sd,n from a variable y 
according to some 
categoricals g1,g2... I wrote the following function:
msd<-function(y,...) {
    print(match.call())
    funy<-function(x) 
list(mean(x,na.rm=T),sd(x,na.rm=T),table(is.na(x))[1])
    gg <- list(...)
    aa<-by(y,gg,funy)
    bb<-matrix(unlist(aa),nrow=3)
    bb
}
this works OK so far there is no empty cell in the crosstab, if there 
is, the unlist statement just run over it and I do not know 
which cell 
is missing, any attempt to label the columns in matrix bb then fail.
Can somebody suggest a method to retrive the list names from 
aa to label 
the columns of the matrix bb?
Thanks for any help
Heberto Ghezzo
McGill University
Montreal Canada