How do I add a static number to \recurselevel?

Question

I currently have:

\dorecurse{8}{

\startcombination[3*3]
{\externalfigure[confcard.pdf][page=\recurselevel, width=98mm]}}
{\externalfigure[confcard.pdf][page=\recurselevel+2, width=98mm]}{}
{\externalfigure[confcard.pdf][page=\recurselevel+4, width=98mm]}{}
{\externalfigure[confcard.pdf][page=\recurselevel+6, width=98mm]}{}
{\externalfigure[confcard.pdf][page=\recurselevel+8, width=98mm]}{}
{\externalfigure[confcard.pdf][page=\recurselevel+10, width=98mm]}{}
{\externalfigure[confcard.pdf][page=\recurselevel+12, width=98mm]}{}
{\externalfigure[confcard.pdf][page=\recurselevel+14, width=98mm]}{}
{\externalfigure[confcard.pdf][page=\recurselevel+16, width=98mm]}{}
\stopcombination
}

Because I've got some incredibly fiddly layout requirements that normal imposition won't handle. (Specifically, double-sided imposition mapping 18 90mmx54mm pages to one double-sided a4 page. I've decided to use external figure, since filterpages won't pipe to combinepages.

However, as I'm trying to solve this in the general case by using dorecurse, how do I manipulate \recurselevel to add a static modifier for each one?

The final layout is

{first side}
[1,3,5]
[7,9,11]
[13,15,17]
{second side}
[6,4,2]
[12,10,8]
[18,16,14]

Answer 1

The answer is not with counters, but with:

\startcombination[3*3]
{\externalfigure[confcard.pdf][page=\the\numexpr 0+\recurselevel\relax, width=98mm]}{}
{\externalfigure[confcard.pdf][page=\the\numexpr 2+\recurselevel\relax, width=98mm]}{}
{\externalfigure[confcard.pdf][page=\the\numexpr 4+\recurselevel\relax, width=98mm]}{}
{\externalfigure[confcard.pdf][page=\the\numexpr 6+\recurselevel\relax, width=98mm]}{}
{\externalfigure[confcard.pdf][page=\the\numexpr 8+\recurselevel\relax, width=98mm]}{}
{\externalfigure[confcard.pdf][page=\the\numexpr 10+\recurselevel\relax, width=98mm]}{}
{\externalfigure[confcard.pdf][page=\the\numexpr 12+\recurselevel\relax, width=98mm]}{}
{\externalfigure[confcard.pdf][page=\the\numexpr 14+\recurselevel\relax, width=98mm]}{}
{\externalfigure[confcard.pdf][page=\the\numexpr 16+\recurselevel\relax, width=98mm]}{}
\stopcombination

The expresion \the\numexpr expression\relax which provides exactly the ability to manipulate math in-document.

Answer 2

Aditya is probably right that this is not the best way to solve an imposition problem, but just for the record: there is also dostepwiserecurse:

\dostepwiserecurse{start}{stop}{step}{commands}