Is 121:3 a promise, or a wish?

The WEB gives it as a promise: "He will not allow your foot to be moved. He who keeps you will not slumber" (121:3). In Hebrew both of its nots are al, which Brown-Driver-Briggs describes as a word of negation "denying however, not objectively as a fact, but subjectively as a wish, expressing therefore a deprecation or prohibition". The next verse uses another Hebrew word for not, lo, twice: "Behold, he who keeps Israel will neither slumber nor sleep" (121:4). So 121:3 can be heard as words said over the one addressed, may he not, with 121:4 then saying it outright; or both verses can be heard, as the WEB gives them, as statements. The Hebrew doesn't settle how 121:3 is best put into English, and these notes don't choose. On either hearing, 121:4 says it without a wish: he "will neither slumber nor sleep".