input: from typing import List
  
  def insert_between(numbers: List[int], separator: int) -> List[int]:
      """ Add a specific number 'separator' between each pair of successive elements in the list `numbers`
      >>> insert_between([], 5)
      []
      >>> insert_between([1, 2, 3], 5)
      [1, 5, 2, 5, 3]
      """
output:     if not numbers:
          return []
      result = []
      for i in range(len(numbers) - 1):
          result.append(numbers[i])
          result.append(separator)
      result.append(numbers[-1])
      return result