This slightly bell-shaped flower has four pedals, overlapping each other like a spread-out deck of cards. It gets 20" high and has a single, alternating green leaf.  At the stem base of and joined to each leaf, a second sprout of leaves comes out from which the flower grows.   Very interesting design.  Grows and spreads in my garden in both partial shade and full sun.   Very hearty and pretty.   What is it called?