text: $semi
  <SQL>
  SELECT * FROM `df` AS `LHS`
  WHERE EXISTS (
    SELECT 1 FROM `df` AS `RHS`
    WHERE (`LHS`.`x` = `RHS`.`x` AND `LHS`.`y` = `RHS`.`y`)
  )
  
  $anti
  <SQL>
  SELECT * FROM `df` AS `LHS`
  WHERE NOT EXISTS (
    SELECT 1 FROM `df` AS `RHS`
    WHERE (`LHS`.`x` = `RHS`.`x` AND `LHS`.`y` = `RHS`.`y`)
  )
  
  