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    $\begingroup$ {a} /. _ -> "who said it works only at the first level?" and {} != 3 vs {} =!= 3. $\endgroup$ Commented Jan 23, 2019 at 14:37
  • $\begingroup$ @Kuba, thank you for your reply. However, I do not fully understand. Apparently in the UnsameQ[] case, the entire {-7,3,2} is becoming x (/after/ the first level?) and not the same as 3. Why does that not continue with Unequal[]? (For that matter, I am a tad stymied why {}!=3 does not return False.) $\endgroup$ Commented Jan 23, 2019 at 15:26
  • $\begingroup$ ! SameQ[] would be {-7, 3, 2} /. x_ /; ! (x === 3) -> w, which returns the same as UnsameQ[]. But ! Equal[] is different than UnsameQ[], as your 2nd and 4th examples show. $\endgroup$ Commented Jan 23, 2019 at 21:00
  • $\begingroup$ Perhaps you want Replace[list, x_ /; x =!= 3 -> w, 1] or Replace[list, x_ /; x =!= 3 -> w, Infinity] $\endgroup$ Commented Jan 23, 2019 at 21:01
  • $\begingroup$ @Michael E2, thank you for your reply. Yes, I also played with !SameQ[]. I see that !Equal[] and UnsameQ[] are different, my questions are: why? and why should I have known that? Also, I would appreciate a pointer to some documentation, explaining why {}!=3 does not return False. Thank you. $\endgroup$ Commented Jan 24, 2019 at 13:23