Forums Scripts & Popups Optional parameters in the middle

Hello

I can't figure this out.
"1 2 3" will always be numbers
"A" will get its value from %calc.A


!calc 1 2 3 A
Result: (1*x + 2*y + 3*z) * A

!calc 1 2 A
Result: (1*x + 2*y) * A

so typing all "1 2 3" should not be necessary, just "1 2" or "1"
(avoiding !calc 1 0 0 A).

How can I do that, as "A" wouldnt always be $$4 ?

Thanks