Argument accessibility in an xmacro

I would like to improve the plugin that I wrote for writing physical quantities according to SI prescriptions. The code I am working on is at

Joris suggested me that I should make arguments editable in the active state of the SI TeXmacs macro that typesets number and units together.

This macro is an xmacro, accepting a variable number of arguments; the first argument is the number and the following ones are the units.

I need still to think up how to organize things (and to figure out how they work!), but I have been able to make a little bit of progress by copying code from the Jolly Writer, pag. 254, last code block.
Now my code for the macro, in the file, is




In this way the fields are editable after activation.
Question: what does <tuple|repeat|1|1> mean as arity?

Thanks in advance,

P.S. maybe the code will change completely once I understand how to make it work; also I am under the impression that the one I wrote now has some defects, in case that will go into its own post