Swell is related to Ocean. Ocean is related to Tide. you are likely to find Tide in/at/on Beach. 
Remain is a different concept from Empty. Empty is related to Vacuum. Machine can be used for Vacuum. 
Swell <--RelatedTo--> Ocean <--RelatedTo--> Tide --AtLocation--> Beach 
Remain <--DistinctFrom--> Empty <--RelatedTo--> Vacuum <--UsedFor-- Machine 