and { only { filter_less { all_rows ; avg attend ; 4000 } } ; eq { hop { filter_less { all_rows ; avg attend ; 4000 } ; indoor year } ; 1983 - 1984 } } = true
select the rows whose avg attend record is less than 4000 . there is only one such row in the table . the indoor year record of this unqiue row is 1983  1984 .