The late Donizetti masterpiece, L'assedio di Calais (The Siege of Calais) is a rarity indeed, even in Europe. Four years after the first performance, l’assedio was not performed again until 1990.  One hundred and eighty-one years after its premiere in 1836, this Glimmerglass production marked the American premiere.  During its composition, Donizetti had struggled with it and bent operatic conventions to seek performances in Paris. Ultimately, the opera was a tactical failure and Donizetti wound up with two versions, with an unequal number of acts. In preparation for this production, Francesca Zambello and Joseph Colaneri worked on a new performing edition that tightened loose ends and yielded a satisfactory, if not compelling, conclusion.  Some ballet music was lost in the cuts, but dance (to curry favor with French opera goers) would be an awkward addition to the nobility and gravity of the plot. In the Zambello/Colaneri conclusion, the final exculpation of six sacrificial hostages was emotionally and musically heartrending.