(man Tk::pack)

Widgets (slaves) have a "packing order" in their parent
("master"). Packing order can be influenced via -before, -after. By
default, widgets are placed at the end of the packing order.

When laying out the widgets, they are processing in packing order.

At each time, a "cavity" is left in which to place the remaining
slaves (at the beginning: cavity == the whole master).

A "parcel" is allocated for the widget:

-  parcel laid out inside the cavity according to the -side option
   (e.g. -side left => parcel height = cavity height, parcel width =
   requested width of slave plus the -ipadx and -padx options)

- size of slave: requested width/height, -ipadx, -ipady. If -fill is
  given (may be none(default), x, y, or both): width/height of parcel,
  minus twice the -pady option

- placement of slave inside the parcel: -anchor, -padx, -pady

- new cavity = old cavity minus parcel


- expansion: -expand option (boolean) in comb. with -side left/right
  or -side top/bottom