and { only { filter_eq { all_rows ; days with frost ; 0 } } ; eq { hop { filter_eq { all_rows ; days with frost ; 0 } ; city / town } ; a coruña } } = true
select the rows whose days with frost record is equal to 0 . there is only one such row in the table . the city  town record of this unqiue row is a coruña .