sub mapn (&@) {
    my ($cb, $n) = splice @_, 0, 2;
    splice @_, 0, 0, (undef)x$n;
    map{
        splice @_, 0, $n; $cb->(@_[0 .. (@_ < $n ? $#_ : $n-1)]);
    } 0 .. (@_/$n)-1;
}

sub mapnz (&@) {
    my ($cb, $n) = splice @_, 0, 2;
    splice @_, 0, 0, (undef)x$n;
    map{
        splice @_, 0, $n; $cb->(@_[0 .. $n-1]);
    } 0 .. (@_/$n)-1;
}