NewGRF and string for accessing a folder

[J0anJosep]

I'm trying to make a NewGRF and I have several folders that have *.png files like "/10/*.png", "/20/*.png", ...
Given a parameter of the NewGRF, I would like to access to each one of those folders. Now I have this kind of code:

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alternative_sprites(spr0, ZOOM_LEVEL_NORMAL, BIT_DEPTH_32BPP) { [ 0, 0, "gfx/20/cursor.png" ] }

I would like to define a string (in this case folder="20"), so I could replace the previous line by something similar to this:

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alternative_sprites(spr0, ZOOM_LEVEL_NORMAL, BIT_DEPTH_32BPP) { [ 0, 0, "gfx/"+folder+"/cursor.png" ] }

I've tried to do it in several ways, but I can't find the correct way to do so. Any help would be appreciated.

[planetmaker]

File names within a spriteset definition cannot be parametrized.

If you want to show one sprite (set) instead of another, you need to define two (or more) sprite sets, one for each parameter value and use a switch to branch to the appropriate sprite set.

[J0anJosep]

Thanks!
I'll write separate .pnml files then.