I finally learned what this plant is. Which I have kept indoors in a pot for two years! It's grown a lot, and I had a hunch it really wasn't a houseplant, but didn't want to put it outside until I figured out what it was.
Except- I still don't know which variety. I don't have a pond, and the two damp places in my yard are in full shade. Well, I planted it out (today is part-cloudy and tomorrow supposed to rain) in the lower part of the yard near a fence. It's a real long chance- doing a transplant at the onset of summer- but the iris certainly wasn't happy in the house anymore. I have been watering it heavily throughout the day. I do hope it survives and may bloom next year, so I can see!