'Build the tree starting from all root level comments.'

def get_initial_candidates(self, comment_tree):

    initial_candidates = comment_tree.tree.get(None, [])
    if initial_candidates:
        offset_depth = min((comment_tree.depth[comment_id] for comment_id in initial_candidates))
    else:
        offset_depth = 0
    return (initial_candidates, offset_depth)
