Select every Nth item from a block of adjacent lines (or from a preexisting set of selections), starting at item 1.
Copy selery.kak somewhere in your autoload directory tree.
Entering :selery <N> in normal mode will allow you to select items 1, 1 + N,
1 + 2N, ..., from an adjacent line block or from a set of multiple selections.
For example, with a 10-line paragraph selected, entering :selery 2 will
select lines 1, 3, 5, 7, 9. With a group of 10 selections, entering :selery 3
will pick selections 1, 4, 7, and 10.
If you often use selery, you may want to bind a normal key to this command or create a short alias to it.
MIT