Task: Question Answering
Input (English): Passage: While Pei and Musho were coordinating the Dallas project, their associate Henry Cobb had taken the helm for a commission in Boston. John Hancock Insurance chairman Robert Slater hired I. M. Pei & Partners to design a building that could overshadow the Prudential Tower, erected by their rival.After the firm's first plan was discarded due to a need for more office space, Cobb developed a new plan around a towering parallelogram, slanted away from the Trinity Church and accented by a wedge cut into each narrow side. To minimize the visual impact, the building was covered in large reflective glass panels; Cobb said this would make the building a "background and foil" to the older structures around it. When the Hancock Tower was finished in 1976, it was the tallest building in New England.
Serious issues of execution became evident in the tower almost immediately. Many glass panels fractured in a windstorm during construction in 1973. Some detached and fell to the ground, causing no injuries but sparking concern among Boston residents. In response, the entire tower was reglazed with smaller panels. This significantly increased the cost of the project. Hancock sued the glass manufacturers, Libbey-Owens-Ford, as well as I. M. Pei & Partners, for submitting plans that were "not good and workmanlike". LOF countersued Hancock for defamation, accusing Pei's firm of poor use of their materials; I. M. Pei & Partners sued LOF in return. All three companies settled out of court in 1981.The project became an albatross for Pei's firm. Pei himself refused to discuss it for many years. The pace of new commissions slowed and the firm's architects began looking overseas for opportunities. Cobb worked in Australia and Pei took on jobs in Singapore, Iran, and Kuwait. Although it was a difficult time for everyone involved, Pei later reflected with patience on the experience. "Going through this trial toughened us," he said. "It helped to cement us as partners; we did not give up on each other.". 
Question: Whose rival erected the Prudential Tower?
Output (English): I. M. Pei & Partners.