% Permitting overload of callbacks is not advised and also not supported
% officially. Don't expect support. It's only there for experiments.

\enabledirectives[system.callbacks.permitoverloads]

\starttext

\startluacode

    callbacks.push("find_data_file",function(whaveter)
        return "whatever"
    end)

    -- If you comment the next line, image inclusion will still work as there is
    -- checking in the figure handlers but when you ask for an existing image it
    -- will crash luatex

    callbacks.pop("find_data_file")
\stopluacode

% \externalfigure[t:/sources/cow.pdf]
\externalfigure[whatever-dummy.pdf]

\stoptext
