Halliday & Matthiessen (2014: 550):
The sequence of clauses within a clause complex is also textually significant as a cohesive domain; in particular, the clause complex licenses certain pattern of ‘ellipsis’ that may involve co-reference. Thus in the paratactic sequence 1 He pointed his arrow, 2 but saw nothing, the Subject of the secondary clause is ‘elliptical’ (but [Subject:] ∅ [Finite/Predicator:] saw [Complement:] nothing), and is interpreted as co-referential with the Subject of the primary clause (he). Similarly, ‘elliptical’ Subjects of non-finite dependent clauses, tend to be interpreted as co-referential with the Subjects of their dominant clauses (as in α I went on to birds β starting with my mother’s feeder). Thus in a clause complex, paratactic and hypotactic co-referential ellipsis may work together to signal the continuity of thematic Subjects, e.g.:||| 1 The scientific community is beginning to recognise the opportunity || 2α but [Subject:] ∅ has done little so far || 2β [Subject:] ∅ to provide useful conceptual tools and means of [[communicating these linkages]] [[[ that can be used || to build the social and political consensus necessary for action]]].