text: In this Friday, March 16, 2018 photo provided by Opera Philadelphia, Marietta Simpson and Frederica von Stade pose at Opera Philadelphia in Philadelphia. Mezzo-soprano von Stade, who turns 73 this summer, will star with Simpson in the Sept. 20 world premiere of "Sky on Swings," an opera about two women afflicted by Alzheimer's disease. The show has been commissioned by Opera Philadelphia from composer Lembit Beecher and librettist Hannah Moscovitch.