Listbox: should unselected single-select options include aria-selected=false?

[craigkovatch]

The current practices doc says:

• In a single-select listbox, the selected option has aria-selected set to true.
• if the listbox supports multiple selection:
  • The element with role listbox has aria-multiselectable set to true.
  • All selected options have aria-selected set to true.
  • All options that are not selected have aria-selected set to false.

I have interpreted this to imply that unselected options in a single-select listbox should have no value for aria-selected, vs multi-select listboxes where all options are expected to have a value, be it true or false.

Is this correct? Either way, it would be good to make this explicit in this document, as the current phrasing leaves this up to interpretation.