:type '(choice (const :tag "No user agent string" nil)
(const :tag "Full" notmuch-mua-user-agent-full)
(const :tag "Notmuch" notmuch-mua-user-agent-notmuch)
:type '(choice (const :tag "No user agent string" nil)
(const :tag "Full" notmuch-mua-user-agent-full)
(const :tag "Notmuch" notmuch-mua-user-agent-notmuch)