How to compress with bzip2 and gzip using xar?

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http://unix.stackexchange.com – In the "Why Xar" page of the Xar google code page it says: "Additionally, this means xar can use different compression methods for each file in the archive. For instance, it might not be a good idea to try to try to compress an already compressed file, but a large file might benefit greatly from using bzip2, whereas a small text file would be better served to use gzip." 1 I read the man page and it does not indicates how to do this. So, does anyone know how? (HowTos)