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Mathematica

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    $\begingroup$ Collect[expr, G[__], h1] /. G :> Composition[h2, G] $\endgroup$ Commented May 11, 2020 at 19:25
  • $\begingroup$ Thanks! that's an elegant way of writing it. However, if I'm not mistaken it still means mathematica has to pattern match the G's "twice", which I was hoping to avoid. Maybe it's just not possible though.. $\endgroup$ Commented May 12, 2020 at 13:36
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    $\begingroup$ Would it make sense to use Collect[...] in that third argument? $\endgroup$ Commented May 12, 2020 at 14:38
  • $\begingroup$ I think (the first) Collect allows me to specify an action on the "coefficients", but no way to obviously act on the "variables", so then I think adding a further Collect cannot directly help. $\endgroup$ Commented May 13, 2020 at 15:06