The difference between English restrictive and nonrestrictive clauses

A nonrestrictive relative clause (henceforth NRR) is shown in (I) and a restrictive relative clause (henceforth RR) in (2).
(1) The swans, which are white, are in that part of the lake.
(2) The swans which are white are in that part of the lake.
Example (1) implies that all the swans under discussion are white. Example (2) implies that the white swans are being distinguished from some other not white swans which are also under discussion. There are many superficial differences between restrictive and nonrestrictive relative clauses; in this paper I show that there is no need for construction-specific stipulations which distinguish between them. The differences arise from the fact that the RR is a modifier, while the NRR is not, and in fact has no syntactic relation to its host/antecedent. Co-indexing (involving a referential index) between the relative clause and its antecedent is central to this account. I examine the requirement that a relative pronoun must have an antecedent, which in the case of a NRR is the sole manifestation of the relationship between the relative clause and its host), and suggest that this holds at a level of discourse structure.