: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)
(const :tag "Emacs" notmuch-mua-user-agent-emacs)
(function :tag "Custom user agent function"
: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)
(const :tag "Emacs" notmuch-mua-user-agent-emacs)
(function :tag "Custom user agent function"