Sentence:  A number of semiempirical estimates by various workers lead to the conclusion that the **f bond becomes symmetric when the **f bond length is about 2.4 to 2.5 A , but aside from the possible example of nickel dimethylglyoxime there have <WSD>been</WSD> no convincing reports of symmetric **f bonds . 
Meaning of the word marked with <WSD> tags:  happen, occur, take place; this was during the visit to my parents' house"