Zoo functions

Hi!

1) The documentation for the zoo package indicates a function lag.zoo .
I can't find it when I try to use it or do a help(lag.zoo). I can find all the 
other zoo functions I tried so far (allthough diff.zoo is missing also).

Any hints?

2) What does the following message imply?

Warning message:
some methods for "zoo" objects do not work if the index entries in 'order.by' 
are not unique in: zoo(rval, x.index[i]) 


Thanks for your help,

Michel