Prose outside the guard, never extracted.
%<*demo>
% A short explanatory comment.
% Raw expl3 names work in \cs{keys_set:nn}, \cs*{__fixture_function:N},
% \cs[fixture sort key]{fixture_name:n}, and \cs{already\_escaped:n}.
% Nested groups work as well: \cs{fixture_{\meta{kind}}:n}.
\newcommand{\demofixture}{demo value}
%</demo>
More prose outside the guard.