Sets the cursor position to an <link linkend="cursor-alphabet">Alphabetic Index</link> into the alphabet active
in this's locale.
After setting the target to an alphabetic index, for example the index for letter 'E', then further calls to cursor_step will return results starting with the letter 'E' (or results starting with the last result in 'D', if moving in a negative direction).
The passed index must be a valid index in the active locale, knowledge on the currently active alphabet index must be obtained using Collator APIs.
Use ref_collator to obtain the active collator for this.
| this |
An BookCache |
| cursor |
The BookCacheCursor to modify |
| idx |
The alphabetic index |