input: Write a python function to generate a recurrence relation given a sequence and initial condition.
  assert generate_recurrence([2, 4, 6, 8], 2) == 4
  assert generate_recurrence([1, 3, 5, 7, 9], 3) == 5
  assert generate_recurrence([10, 20, 30], 5) == 15
output: def generate_recurrence(sequence, initial):
      if initial >= len(sequence):
          return None
      return sequence[initial]