How to store lengths in variables

[Urs Liska]

Hi,

I would like to parametrize my paper settings. But for that I need to 
either store the measurement unit in a variable (outside the paper 
block) or store the plain number in the variable and create a length 
from that within the paper block.

tm = 5\in

\paper {
  top-margin = #tm
}

This doesn't work because LilyPond doesn't recognize the "\in" in the 
toplevel variable definition.

And rather than using the LilyPond syntax I would need to store the 
settings in an alist like

values =
#'((top-margin . 5)
   (bottom-margin . 2.5))

etc. and retrieve those values in the paper definition.

So I need to either store the numbers together with their unit (so I can 
directly use them in the paper block) or have a way to "create" that 
variable from the stored number within the paper block.

Any pointers appreciated
Urs