Easy question: ¿How to skip a parameter and set it to default?

Hello there, this might seem like a incredibly naive question, nonetheless I haven't found an answer by myself so I require your help:

When setting the panorama to load a grid (not a picture) I want to set some parameters to default (just skip them and go to the next one), I thought that by writing the word "default" it would be enough, but when I load up the panorama through the html file I get a black screen. I tried writing " " instead inside the commas but it didn't work either.

So, basically the line would be like this:

<krpano version="1.0.8"><preview type="grid(cube, default {this is the parameter I want to skip and set to default but don't know how}, 15);" details="16"/></krpano>

I think it should be fairly easy to do this, and important too because I can't imagine how useful it should to be to do so, perhaps you would notice that I have never programmed in my life but I'm intent on learning.

THANK YOU ALL!

Thank you for the reply, I actually came up with my doubt after reading that, I still cannot find an answer to my question, If I set the parameters:

<krpano version="1.0.8.15" preview.type="grid(,40,40)"> </krpano>

I get a working xml file and I'm able to load a cube grid preview with 40 x 40 squares, this is what confuses me, because I just left the part where I should specify the type of grid (Cube or Sphere) in blank and it automatically called up a cube for the preview, and it didn't tell me there was any error. Now, If I try to apply the same logic to the next parameter, the xsteps, say, I write this:

<krpano version="1.0.8.15" preview.type="grid(cube,,40)"> </krpano>

I don't get a working .xml file.

If the same logic applied it would call for the defaul amout of xsteps, or so I think it should. I've tried to put "" and 'default' with no satisfactory results. Maybe there's just no way to skip a parameter setting it to default automatically. ¿Any ideas?