For any positive integer n, define d(n) to be n plus the sum of the digits of n. For example, d(75) = 75 + 7 + 5 = 87. The number n is called a generator of d(n). Some numbers have more than one generator: for example, 101 has two generators, 91 and 100. A number with no generators is a self-number. There are thirteen self-numbers less than 100: 1, 3, 5, 7, 9, 20, 31, 42, 53, 64, 75, 86, and 97. Write a function that returns the number of positive self-numbers less than a number, threshold.
