+ "Set the function used to query passphrase.
+
+PASSPHRASE-CALLBACK is either a function, or a cons-cell whose
+car is a function and cdr is a callback data.
+
+The function gets three arguments: the context, the key-id in
+question, and the callback data (if any).
+
+The callback may not be called if you use GnuPG 2.x, which relies
+on the external program called `gpg-agent' for passphrase query.
+If you really want to intercept passphrase query, consider
+installing GnuPG 1.x _along with_ GnuPG 2.x, which does passphrase
+query by itself and Emacs can intercept them."