@dx = qw<
    -0.533  0.270  0.859 -0.043 -0.205 -0.127 -0.071  0.275
     1.251 -0.231 -0.401  0.269  0.491  0.951  1.150  0.001
    -0.382  0.161  0.915  2.080 -2.337  0.034 -0.126  0.014
     0.709  0.129 -1.093 -0.483 -1.193  0.020 -0.051  0.047
    -0.095  0.695  0.340 -0.182  0.287  0.213 -0.423 -0.021
    -0.134  1.798  0.021 -1.099 -0.361  1.636 -1.134  1.315
     0.201  0.034  0.097 -0.170  0.054 -0.553 -0.024 -0.181
    -0.700 -0.361 -0.789  0.279 -0.174 -0.009 -0.323 -0.658
     0.348 -0.528  0.881  0.021 -0.853  0.157  0.648  1.774
    -1.043  0.051  0.021  0.247 -0.310  0.171  0.000  0.106
     0.024 -0.386  0.962  0.765 -0.125 -0.289  0.521  0.017
     0.281 -0.749 -0.149 -2.436 -0.909  0.394 -0.113 -0.598
     0.443 -0.521 -0.799  0.087>;

@dy = qw<
     0.136  0.717  0.459 -0.225  1.392  0.385  0.121 -0.395
     0.490 -0.682 -0.065  0.242 -0.288  0.658  0.459  0.000
     0.426  0.205 -0.765 -2.188 -0.742 -0.010  0.089  0.208
     0.585  0.633 -0.444 -0.351 -1.087  0.199  0.701  0.096
    -0.025 -0.868  1.051  0.157  0.216  0.162  0.249 -0.007
     0.009  0.508 -0.790  0.723  0.881 -0.508  0.393 -0.226
     0.710  0.038 -0.217  0.831  0.480  0.407  0.447 -0.295
     1.126  0.380  0.549 -0.445 -0.046  0.428 -0.074  0.217
    -0.822  0.491  1.347 -0.141  1.230 -0.044  0.079  0.219
     0.698  0.275  0.056  0.031  0.421  0.064  0.721  0.104
    -0.729  0.650 -1.103  0.154 -1.720  0.051 -0.385  0.477
     1.537 -0.901  0.939 -0.411  0.341 -0.411  0.106  0.224
    -0.947 -1.424 -0.542 -1.032>;

sub mean   { my $s; $s += $_ for @_; $s / @_ }
sub stddev { sqrt( mean(map { $_**2 } @_) - mean(@_)**2) }

@rules = (
sub { 0 },
sub { -$_[1] },
sub { -$_[0] - $_[1] },
sub {  $_[0] + $_[1] }
);

for (@rules) {
    print "Rule " . ++$cnt . "\n";

    my @ddx; my $tx = 0;
    for my $x (@dx) { push @ddx, $tx + $x; $tx = &$_($tx, $x) }
    my @ddy; my $ty = 0;
    for my $y (@dy) { push @ddy, $ty + $y; $ty = &$_($ty, $y) }

    printf "Mean    x, y  :%7.4f%7.4f\n",   mean(@ddx),   mean(@ddy);
    printf "Std dev x, y  :%7.4f%7.4f\n", stddev(@ddx), stddev(@ddy);
}