gtk:header-bar-pack-start (header child)

header -- a gtk:header-bar widget
child -- a gtk:widget child widget to be added to the header bar

Adds a child widget to the header bar, packed with reference to the start of the header bar. Code fragment for a header bar with two buttons. The Cancel button is placed on the left side and the Done button on the right side of the header bar.

(let (...
      (header (make-instance 'gtk:header-bar
                             :show-title-buttons nil))
      (button (make-instance 'gtk:button
                             :label "_Done"
                             :use-underline t
                             :sensitive nil))
      (cancel (make-instance 'gtk:button
                             :label "_Cancel"
                             :use-underline t))
      ...)
  ...
  (gtk:header-bar-pack-start header cancel)
  (gtk:header-bar-pack-end header button)
  ... )