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