Question: When is `print/diff` used, and how do I debug it?

Both eval and showstat show that a procedure `print/diff` exists, but I am having trouble figuring out when the procedure is invoked. I expect it to be invoked---like any other `print/` procedure---when diff is output as an unevaluated function call. So I enter,

diff(f(x), x$2);

This does not produce any debug output. Why? Checking with lprint shows that the result does indeed contain diff as an unevaluated function call.

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