AngularJs UI预先匹配前导字符

The typeahead functionality in AngularJs UI seems simple and powerful however I have been trying to figure out how to get the matching to be done on the leading characters. For example if I type 'A' in the input box I would like to see all the states that start with 'A" and not all the states that contain an 'A' in the name. I have been looking for a couple of days on this and it seems that Angular has the concept of a custom filter that has a 'comparator'. The docs on this have a simple example that does not show the exact syntax for implementing a comparator. The typeahead functionality in AngularJs UI see