However, e.g., Haliday & Hasan (1976) stress the importance of anaphoric links as a cue for coherence structures.
(a) is the first segment and (c) is the second segment of what is actually one single discourse segment, separated by the intervening discourse segment (b), which is in an attribution relation with (a) (and therefore also with (c), since (a) and (c) are actually one single discourse segment).