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sub mapn (&@) {
    my ( $callback, $n ) = splice @_, 0, 2;
    map $callback->( splice @_, 0, $n ), 0 .. @_ / $n -1;
}

sub mapnz (&@) {
    my ( $callback, $n ) = splice @_, 0, 2;
...
        $callback->( @_[ $i .. ( $j += $n ) - 1 ] );
    } 0 .. @_ / $n - 1;
}

sub mapn (&@) {
    my ( $callback, $n ) = splice @_, 0, 2;
...
        $callback->( splice @_, 0, $n );
    } 0 .. @_ / $n - 1;
}

sub mapn (&@) {
    my ( $callback, $n ) = splice @_, 0, 2;
...
        $callback->( @_[ $i .. ( $j += $n ) - 1 ] );
    } 0 .. @_ / $n - 1;
}

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