Language has evolved to convey information as efficiently as possible, and as a result avoids lengthy identifying descriptions and other lengthy phrasings where shorter ones will do. One aspect of this tendency towards “shorthand” is seen in anaphora, the phenomenon of coreference between an earlier, potentially more descriptive NP and a later anaphoric pronoun or definite NP (with a determiner like the or these). (The reverse sequencing, cataphora, is seen occasionally as well.)  Coreference is yet another source of ambiguity in language, scarcely noticeable by human language users (except in ambivalent cases such as “When Flight 77 hit the Pentagon's wall, it disintegrated”), but problematic for machines.