function twentynineteen_discussion_avatars_list()

So I’m going through all the theme’s core files and trying to figure out where a specific function is at…

In twentynineteen-master/template-parts/post/discussion-meta.php I’m looking at the following line:

if ( $has_responses ) {
twentynineteen_discussion_avatars_list( $discussion->authors );
}

ok great….. I have searched the entire theme and can’t find where the function twentynineteen_discussion_avatars_list() is created….. am I missing something? Can anybody help me?