[NTG-context] Customising referencing failures (\in, etc)

[Robin.Kirkham at csiro.au]

Folks,

Is there a way to customise the ?? that is substituted in the output when a reference does not work out (nonexistent reference label, etc)?  I’d like to make it more obvious for proofreading (like making it red, putting a marker in the margin, or something).

Cheers, Robin

\starttext
\input tufte
Bad reference \in[missing].
\input tufte
\stoptext