CF995F Cowmpany Cowmpensation

Allen, having graduated from the MOO Institute of Techcowlogy (MIT), has started a startup! Allen is the president of his startup. He also hires $ n-1 $ other employees, each of which is assigned a direct superior. If $ u $ is a superior of $ v $ and $ v $ is a superior of $ w $ then also $ u $ is a superior of $ w $ . Additionally, there are no $ u $ and $ v $ such that $ u $ is the superior of $ v $ and $ v $ is the superior of $ u $ . Allen himself has no superior. Allen is employee number $ 1 $ , and the others are employee numbers $ 2 $ through $ n $ . Finally, Allen must assign salaries to each employee in the company including himself. Due to budget constraints, each employee's salary is an integer between $ 1 $ and $ D $ . Additionally, no employee can make strictly more than his superior. Help Allen find the number of ways to assign salaries. As this number may be large, output it modulo $ 10^9 + 7 $ .

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In the first sample case, employee 2 and 3 report directly to Allen. The three salaries, in order, can be $ (1,1,1) $ , $ (2,1,1) $ , $ (2,1,2) $ , $ (2,2,1) $ or $ (2,2,2) $ . In the second sample case, employee 2 reports to Allen and employee 3 reports to employee 2. In order, the possible salaries are $ (1,1,1) $ , $ (2,1,1) $ , $ (2,2,1) $ , $ (2,2,2) $ , $ (3,1,1) $ , $ (3,2,1) $ , $ (3,2,2) $ , $ (3,3,1) $ , $ (3,3,2) $ , $ (3,3,3) $ .