Required children of role=combobox

A role=combobox does not require a role=textbox as a child but  
requires one role=listbox as a child? Please clarify how role=combobox  
is supposed to work--in particular how the text entry part gets  
implied by the role itself (if that's the case as it seems).

-- 
Henri Sivonen
hsivonen@iki.fi
http://hsivonen.iki.fi/