`?` is not the only option

if there’s one thing I learned from Go…

let mut list = WalkList::new(args);

let condition = list.expect_arg(c, src, "missing `if` condition");
let if_true = list.expect_arg(c, src, "missing `if` true branch");
let if_false = list.expect_arg(c, src, "missing `if` false branch");
list.expect_nil(c, src, "extra arguments after `if` false branch");

if !list.ok {
    return Ok(());
}

…it’s that ? is not the only option.