class C {

    has Int  $.i;
    has Bool $.b;

    submethod BUILD (*%init) {
	$.i = 0;
	$.b = False;

	?%init{"i"} && %init{"b"} and die "Don't be insane!";
	for keys %init -> $attr {
	    ...
	    }
	}

    method is-insane () returns Bool {
	?$.i && $.b;
	}
    }

my $c = C.new (i => 1, b => True);
$c.is-insane.perl.say;
