World: r3wp

in the foo function /something is a refinement and not an _optional_ 
refinement. In your my-sum function amount is not a refinment and 
my-sum 1 2 none == 3 is correct. What I am saying is that the extra 
none! adds polymorphism (but i have not investigated that too much 
so i could be mistaken), so you can write: sum 1 2 or sum 1 2 3 or 
sum 1 2 none without checking for none before calling the function.

Ehm, the nature of refinements is that they're optional

And you can't write SUM 1 2 without a refinement if you have an extra 
AMOUNT parameter. The number of parameters is fixed in REBOL. The 
way to have optional parameters is to use refinements (or a BLOCK! 
argument)

I don't see the benefit to the propsal at this point.