[julia-users] Writing a mutable function (exclamation mark function)

[dworkg1]

I'm currently trying to understand how functions with an exclamation mark 
at the end work. I know the exclamation mark is just a notational point 
however I'm currently confused at how to actually write a mutable function.
If a = 1

function add_one(a)
    return a + 1
end

running add_one(a) twice outputs:
    2
    2

how would I create add_one!(a) to output:
    2
    3