How do I detect if an optional keyword was supplied to a declarative macro?

Question

I'm writing a macro:

macro_rules! foo {
    ($(print)?) => {
        // run `println!("hello") if print is given
    }
}

Which could be called as:

• foo!() which would do nothing

• foo!(print) which would print hello

How do I detect if print was supplied? When I use a repetition operator I need to put a variable in. Is there some sort of empty variable I can use? ((print $print:empty)?)