input: Write a python function to calculate n choose r modulo p.
  assert combination_modulo(10, 2, 13) == 6
  assert combination_modulo(15, 12, 43) == 25
  assert combination_modulo(17, 9, 18) == 10
output: def combination_modulo(n, r, p):
      if r == 0:
          return 1
      num = den = 1
      for i in range(r):
          num = (num * (n - i)) % p
          den = (den * (i + 1)) % p
      return (num * pow(den, p - 2, p)) % p