An `open-line' command.

An `open-line' command.

/**
 * Replace the current region with modifier(selection). Deactivates region and
 * sets point to the end of the inserted text, unless keep_point is true, in
 * which case the point will be left at the beginning of the inserted text.
 */
function modify_region(field, modifier, keep_point) {

/**
 * Given a callback func and an interactive context I, call func, passing either
 * a focused field, or the minibuffer's input element if the minibuffer is
 * active. Afterward, call `ensure_index_is_visible' on the field. See
 * `paste_x_primary_selection' and `open_line' for examples.
 */
function call_on_focused_field(I, func) {