[xPand] Express Derivatives wrt to Time wrt to the log of the Scale Factor a

[Kay Lehnert]

I am working with xPert and xPand and would like to re-express time derivatives as derivatives with respect to ln(a), i.e. d/d ln a = (1/Ha)*(d/d t). For example, if I have the equation of motion of a scalar field like

eom0 = CD[\[Alpha]][CD[-\[Alpha]][sf[LI[0], LI[0]]]] -  D[V[sf[LI[0], LI[0]]], sf[LI[0], LI[0]]] - nf[LI[0], LI[0]] D[m[sf[LI[0], LI[0]]], sf[LI[0], LI[0]]]

and I express it with MyxPand[expr_, gauge_, order_] :=

 ToxPand[expr, dg, u, du, h, gauge, order]

in a metric-dependent way, I would like to be able to have the time derivatives replaced.

Any hints that point me in the right direction are highly appreciated. Thank you very much in advance for your help. :)