A man hiking in a heavily wooded area of northern New Jersey was killed by a black bear during the weekend, the police said on Monday, in what experts called an extremely rare attack.
问：in what experts called an extremely rare attack.中填what，答案说是填what ，作called的宾语，那也就是call sth/sb sth： “把某物称作什么 ”也就是“专家把这次事件称作一次最罕见的袭击事故。”所以用what 一词指代前文所言的熊袭击人的事件，那为何用了in what 还放在了主语experts之前？
what 是“融合性关系代词”，依据语境，what = an attack that。 in what experts called an extremely rare attack.= in an attack that experts called an extremely rare attack. what 作 called 的宾语，an extremely rare attack 作宾补。
1. 最简单的表达是： in an attack（在一次袭击事件中）
2. 把介词短语扩展，增加定语，得到：in an attack that experts called an extremely rare attack.
3. 把 an attack that 用 what 替换：in what experts called an extremely rare attack.
