/inc doesn't make a var global as long as the var was set up previously.
The use of []'s like that isn't really the best way to script it anyhow. That's similar to using GOTO instead of WHILE. Although it works, WHILE (or $+() in your example) is the better choice. = is just a better way to script it. It makes it clear that it's not a global variable and clearly shows what part is the data being put into the variable. Much better if you're showing someone who is new to scripting how to do it.
What it comes down to is that when you're teaching someone (most answers on this forum will be teaching the OP how to script something), you should keep the code as clean as possible and stick to normal scripting practices rather than shortcuts unless you're also taking time to explain that it is a shortcut and why you're using it.
In all learning, shortcuts should come after learning how to do it the normal way. It prevents confusion later. It's one thing if you're teaching a shortcut that makes a major difference in speed. It's quite another if you're showing a preference in how the script should look.
For example, some people prefer to script everything on one line using |'s. Although that will work, if you're teaching someone how to do something, you should show it on multiple lines. Although one line coding is a preference and can be used by anyone who likes that method, it's not the way to teach scripting. Using or not using ='s and using or not using $+() are also preferences and even if they don't confuse the issue as much as one line scripting, it's better to stick to a normal method to avoid confusing a new scripter. Just something to keep in mind.
It's up to you what you want to do. But you definitely don't need to use $calc() there instead of either /inc or just +1. Just trying to point out that using = is how /var is meant to be used.
