Sir; Even in these days of convenience foods, putting tuna in a pouch seems a pretty revolutionary idea. (The Grocer, last week, p6.) So it will be interesting to see how Princes Foods fares with the development. But will it, I wonder, signal the beginning of the end for the humble can? There have been predictions for years that the days of the can were numbered. But it's still the most effective, and safest, form of packaging. J Silverstone Woking {{LETTERS }}