Method 'a' not found for invocant of class 'Mu'

gistfile1.txt

	role MyAtPos[$s,$i] {
		method at_pos($j) {
			Proxy.new:
				FETCH => method {$s.a[$i][$j]},
				STORE => method ($p) {
					$s.h{ $s.a[$i][$j] } = 0;
					$s.h{ $p           } = [$i, $j];
					$s.a[$i][$j]=$p;
				};
		}
	}
	method at_pos(Board $s: $i) {
		Proxy.new:
			FETCH => method {
				$s.a[$i] does MyAtPos[$s,$i];
			},
			STORE => method ($p) {die "Err: Can't assign to an entire row"};
	};