The is-ought gap

In another thread, A.N. Prior's attempted refutation of the is/ought gap came up. Here's one of Prior's examples:

a. Tea-drinking is common in England.
b. Therefore, tea-drinking is common in England, or all New Zealanders ought to be shot.

(a) entails (b), (a) is descriptive, and allegedly, (b) is evaluative.

There's an unjustly neglected article by Toomas Karmo ("Some Valid (but no Sound) Arguments Trivially Span the Is-Ought Gap", Mind [1988]: 252-7) that responds to this sort of example and provides a general proof of a version of the is-ought gap. Karmo's argument goes roughly like this: