Handing back a borrow

# Handing back a borrow (owner, 2026-09-29): `-> lent @x T`. `@x` names a lifetime group, announced
# after the function's name (owner, 2026-10-02): everything marked `@x` lives and dies together, so the result of `longer`
# is good while both 'a and 'b are; `pick`'s result only needs 'a. Like Rust's `'x`, spelt with
# `@` since `'` means a variable here. A borrow of something that dies at the return is refused.
# The group may be left out (`first`), and the prover works it out from the body (a `#strict` file
# had to write it, until `#strict` went on 2026-10-07).
fn longer [@x] [lent @x 'a': str.utf8, lent @x 'b': str.utf8] -> lent @x str.utf8 {
    if str.bytes[lend 'a'] at_least str.bytes[lend 'b'] { return lend 'a'; }
    return lend 'b';
}
fn pick [@x, @y] [lent @x 'a': str.utf8, lent @y 'b': str.utf8] -> lent @x str.utf8 {
    return lend 'a';
}
fn first [lent 'a': str.utf8] -> lent str.utf8 {
    return lend 'a';
}
MAIN {
    let 's' := "hello";
    let 't' := "hi";
    let 'l' := longer[lend 's', lend 't'];
    let 'p' := pick[lend 't', lend 's'];
    let 'f' := first[lend 's'];
    std::print.useful['l', " ", 'p', " ", 'f', newln];
}