Prose outside the guard, never extracted.
%<*demo>
% # ASCII *markup*
% ## Inline elements
% *Bold*, _italic_, `code_value`, `\raw_name`, and \keys_set:nn.
% An escaped \*asterisk\*, an escaped \_underscore\_, and plain_name.
% ====
% - First item mentions \foo.
% * Second item contains `x_y`.
% Text after the list.
%% This source-code comment remains in the listing.
\newcommand{\asciifixture}{demo value}
%</demo>