Task: Question Answering
Input (English): Passage: "Climbing over rocky mountain" is the best known piece from Thespis, as it was transplanted in 1879 into one of Gilbert and Sullivan's most successful operas, The Pirates of Penzance. In 1902, Gilbert told a correspondent that this had happened accidentally. He and Sullivan had arrived in New York to produce the new opera, but the composer discovered that he had left his sketches behind in England. Fortunately, the entrance chorus from Thespis fitted the situation almost exactly, so it was substituted instead.Several scholars have doubted that explanation. In Sullivan's autograph score for the later work, the first part of "Climbing over rocky mountain" is actually taken from a Thespis copyist score, with the Thespis words cancelled and the new words written in, which raises the question of why Sullivan had a Thespis score to hand, if not for that purpose.Some suggest that other music from Thespis could have been used in Pirates. Goldberg suggests that "It is reasonable to believe that Sullivan made generous use of his Thespis music in other operettas: perhaps owing to the circumstances under which The Pirates of Penzance was written, it contains more than one unacknowledged borrowing from the unlucky firstling of the lucky pair." Reginald Allen says that "it seems certain" from its "rhythmic structure" that part of the Act I finale of Thespis, "Here's a pretty tale for future Iliads and Odysseys" became the original Act II finale in Pirates, "At length we are provided with unusual felicity", which was later deleted. Tillett and Spencer propose that most of Act I of Pirates was taken from Thespis. However, there is only circumstantial evidence for these suggestions. Except for "Climbing over rocky mountain", neither author admitted to borrowing from Thespis for later works. 
Question: What composer discovered that he had left his sketches behind in England?
Output (English): Gilbert.