# arg_1 to arg_n specify size in dimension k,
# last arg is a reference to a sub
# - expecting n arguments x1 ... xn
# - returning the desired value at $na->[x1]...[xn]
sub narray {
    my $size = shift;
    my $val = pop;
    my $na;
    for my $i (0 .. $size - 1) {
        $na->[$i] = @_ ? narray(@_, sub {&$val($i, @_)}) : &$val($i);
    }
    $na;
}