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How do you decorate or annotate a variable in a replacement rule?

For example

{(e[(-X + Y)^2 || f])} //. {e[(-a_ + b_)^2 || f] -> (-a + b)^2}

Would ideally return:

(-lagXdt + lagYdt)^2

Update: The following does not return lagXdt and lagYdt but rather lagadt and lagbdt:

{(e[(-X + Y)^2 || f])} //. {e[(-a_ + b_)^2 || 
     f] -> (-Symbol["lag" <> ToString@a <> "dt"] + 
      Symbol["lag" <> ToString@b <> "dt"])^2}
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    $\begingroup$ Your code sample is ... quite wrong. You should balance your parentheses and check what the // was supposed to be. $\endgroup$ – MarcoB Dec 12 '19 at 22:18
  • $\begingroup$ Thanks @MarcoB. Now fixed. $\endgroup$ – Hedgehog Dec 12 '19 at 22:51
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Got it eventually, the key was to use :> instead of ->:

{(e[(-X + Y)^2 || f])} //. {e[(-a_ + b_)^2 || f] :> (-Symbol["lag" <> ToString@a <> "dt"] + Symbol["lag" <> ToString@b <> "dt"])^2}

{(-lagXdt + lagYdt)^2

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