for (0..3) {
        print "mapat $_ f: ",
              dumpf(mapat($_, \&f, $mats)), $/;
    }
    # mapat 0 f: f([[[1,2],[3,4]],[[5,6],[7,8]]])
    # mapat 1 f: [f([[1,2],[3,4]]),f([[5,6],[7,8]])]
    # mapat 2 f: [[f([1,2]),f([3,4])],[f([5,6]),f([7,8])]]
    # mapat 3 f: [[[f(1),f(2)],[f(3),f(4)]],[[f(5),f(6)],[f(7),f(8)]]]