As the nuns took their final steps, ascending the scaffold where the guillotine sat sharp and gleaming, a moment of inspiration took hold. The sisters started to sing, each one continuing the hymn the other had started, as one by one they met their death. That’s how Francis Poulenc’s powerful opera “Dialogues of the Carmelites” ends — but it’s not a fiction constructed for dramatic effect. A similar scene played out in 1794, when the real-life Carmelite martyrs faced execution during the French Revolution. The 29-year-old novice Marie-Geneviève Meunier was the first in line to die on July 17, 1794, when 16 members of a convent from Compiègne were condemned to death for their faith. As the prison cart passed through the Paris streets, the nuns sang their hymns. But at the foot of the guillotine, it was the youngest among them — Meunier — who initiated their final song. She had taken the name Sister Constance. Her story most resembles that told in “Dialogues of the Carmelites:” A young woman, she found herself shaken by the dangers posed by the French Revolution. Her family, concerned for her wellbeing, even sent her brother to fetch her home from the convent. But she stood with her sisters — until the end. But the opera version differs from real life in significant ways, not least in the final song the nuns chose to sing. On stage, the Carmelites chant the Salve Regina hymn. In real life, Sister Constance chose Psalm 117 — Laudate Dominum — instead. See how composer Francis Poulenc brought this harrowing true story to the stage in “Dialogues of the Carmelites,” coming this fall: sfopera.com/carmelites (📸: Ron Scherl for 1982's production)