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    $\begingroup$ Your Fold[#1-#2&,a b] is an amusing trick, but probably difficult to understand for people that are new to Mathematica. $\endgroup$ Commented Sep 26, 2022 at 14:19
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    $\begingroup$ @user293787 I remember a time when it was not even on my radar. $\endgroup$ Commented Sep 26, 2022 at 14:24
  • $\begingroup$ why does the Fold[#1 - #2 &, a b] term have the same function as (t1 . Mi . t2) ? $\endgroup$ Commented Sep 26, 2022 at 16:50
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    $\begingroup$ Focus on the first term #1 and the second term #2 of the element-wise product a b. Do a #1-#2&, you get: qm[0] qn[0] - qm[1] qn[1]. Now this becomes the first term. The second term is the next one in the list. When it folds, you get qm[0] qn[0] - qm[1] qn[1] - qm[2] qn[2]; and so it goes until the end of the list. You can study more examples from the docs. Change Fold to FoldList to see the steps. $\endgroup$ Commented Sep 26, 2022 at 16:58