X/(1-q) Plethystic Substitution

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Hello everyone!

I'm trying to perform the plethystic substitution $f \rightarrow f[\frac{X}{1-q}]$ (which is given by $ f[\frac{X}{1-q}] = f(x_1,qx_1,q^2x_1,...,x_2,qx_2,...)$)

I tried to implement this in Sage using the command .plethysm(X/(1-q)), but it doesn't seem to be giving me the right thing (I have no idea how it's interpreting 1/(1-q) actually). Does anyone know if this is implemented, and if so, how to get it to work?

Thanks so much in advance!