With no arguments, focus returns the
name of the widget to which keyboard events
are currently delivered, known as the
keyboard focus.
When given an argument,
focus changes the keyboard focus.
If what is the name of a widget, that
widget will be made the keyboard focus.
If what is next, the keyboard focus
will be changed to the next item in the
focus order for that window. If what is
previous, the keyboard focus will be
changed to the previous item in the focus order
for that window. The focus order is determined
automatically by Tk.