gistfile1.txt

    # in Rakudo you can't pass a Mu to where an Array is expected,
    # so we add multis for explicit undefined values
    multi sub mirror(Any:U $a, Any:U $b) { return True;  }   #OK not used
    multi sub mirror(Any:U $a, @b)       { return False; }   #OK not used
    multi sub mirror(@a,       Any:U $b) { return False; }   #OK not used

    multi sub mirror(@first, @second) {
        if (@first|@second == (Mu,)) {
            return @first == @second ;
        }
        mirror(left(@first),right(@second)) and mirror(right(@first),left(@second))
    }

    ok mirror(Mu,Mu),"mirror works with empty trees"; # fails