+(defcustom notmuch-show-header-line t
+ "Show a header line in notmuch show buffers.
+
+If t (the default), the header line will contain the current
+message's subject.
+
+If a string, this value is interpreted as a format string to be
+passed to `format-spec` with `%s` as the substitution variable
+for the message's subject. E.g., to display the subject trimmed
+to a maximum of 80 columns, you could use \"%>-80s\" as format.
+
+If you assign to this variable a function, it will be called with
+the subject as argument, and the return value will be used as the
+header line format. Since the function is called with the
+message buffer as the current buffer, it is also possible to
+access any other properties of the message, using for instance
+notmuch-show functions such as
+`notmuch-show-get-message-properties'.
+
+Finally, if this variable is set to nil, no header is
+displayed."
+ :type '(choice (const :tag "No header" ni)
+ (const :tag "Subject" t)
+ (string :tag "Format")
+ (function :tag "Function"))
+ :group 'notmuch-show)
+