New Protocol

use Protocol;

protocol FormattableKeysProtocol {
   method keys { ... }
   method formatted-keys { $.keys.join: ", " }
}

constant FormattableKeys = FormattableKeysProtocol.coerce;

my FormattableKeys $a = %( a => 1, b => 2 );
say $a;
say $a.formatted-keys;

multi sub a(FormattableKeys $a) {
    say $a + 1;
    say $a.formatted-keys;
    say $a.^name
}
a %(x => 1, y => 2, z => 3);