input: 
  from typing import Callable
  
  
  def sum_using_lambda(a: int, b: int) -> Callable[[int, int], int]:
      """ Generate a lambda function that can sum two numbers.
      >>> sum_using_lambda(3, 4)(3, 4)
      7
      """
      return lambda a, b: a + b
  
output:     return lambda x, y: x + y