unidentified tank

Can anyone tell me what the plastic approx 1 gallon tank located in the passenger wheel well is for? I cannot find any schematics on it. Looks like some kind of overflow tank. It is not the aluminum tubular one. Called Toyota dealer parts, teenage guy had a disgusted attitude, told me to hang up and have a man call after insisting it was "just the windshield wiper fluid lady". I assure you, it is not. Second dealer thought it was the inverter tank, but come on, that is not only on the top right but clearly identifiable. Wants me to take it in to their supervisor for a look-see after asking around, as nobody at the second Toyota dealership seems to know, while telling me they are the only ones who know the prius. Seems to be a simple question. Anyone have a manual?

 

Thanks for the replies. I figured people in this forum would have the answer.