premise: If the **f bond is linear then there are three reasonable positions for the <mask> : ( 1 ) The <mask> are centered and hence all lie on a sheet midway between the oxygen sheets ; ( 2 ) all <mask> lie on a sheet , but the sheet is closer to one oxygen sheet than to the other ; ( 3 ) <mask> are asymmetrically placed , either randomly or in an ordered way , so that some <mask> are closer to the upper oxygen atoms while others are closer to the lower oxygen atoms .
hypothesis: the proper context for <mask> here should be an atom of hydrogen