confused understanding in paper

[songtaoshi]

Hi dear researchers, I have some trouble understanding the following sentences in the original paper.
"To calculate these baselines, we first detect candidate antecedents by finding all mentions of PERSON entity type, NAME mention type (headed by a proper noun), and, for structural cues, that are not in a syntactic position which precludes coreference with the pronoun."

the last part makes me really confused and i cannot get i across.