Japanese double negatives

いり口で荷物を　あずけては　　　いけません。
Literally: Leaving your luggage at the entrance is not okay.

入り口で荷物を　あずけなくては　いけません。
Literally: NOT leaving your luggage at the entrance is not okay.

In Japanese, the 2nd sentence means the same thing as the first, but with more emphasis added by the double negative, i.e. the meaning (non-literal) of the first "Don't leave your luggage by the entrance." becomes "You MUST not leave your luggage by the entrace!" with the double negative added.
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(updated 11/15/05)

Oops. Actually the second sentence does not mean the same as the first one.

1st: DON'T leave your luggage by the entrance.
2nd: You MUST leave your luggage by the entrance.

Double-negatives actually changes a request/command into an imperative. I guess this isn't so different from the literal translation. Another example:

行かなければ = If I did not go
ならない = it does not become, it isn't the case that.
行かなければならない = It isn't the case even if I didn't go = I must go.

It is interesting that in Japanese there doesn't seem to be another way of saying "must" other than this double-negative form (with just one negative verb you say "must not") using しなければならない or しないではいけない (both mean it is not okay that I/you/one does not do, or I/you/one must do).