[help] Define a variable from its $arg


1) Give a description of the problem
I have a piston that only run from url. I use this piston to update variable. For now, I use multiple lines to check: if $arg.x not empty then set variable x to $arg.x and so on for each variable that is likely to be updated from url…

I would like to use something like this: set variable {$arg.x} to $arg.x
so if the arg match a variable it would update it automatically and if not matching I could set a notification with the $arg.x like this: $arg.x has not been updated, check if the variable exist

2) What is the expected behaviour?
set any variable in the same line

3) What is happening/not happening?
in set variable option, I can only select variable inside drop down list… Could be fun to have expression to define the variable or something