input: Write a function to describe the use of recursion in solving a given problem in Python. Return a string that explains the concept of recursion and how it is used in Python. If the input string is empty, return an empty string.
  assert recursion_explanation("How do we use recursion to solve a given problem in Python?")=="Recursion is a powerful technique for solving problems. It involves defining a problem in terms of itself. A recursion problem is solved by breaking it down into smaller problems, until the problem is small enough to be solved directly. The result from a smaller problem can then be combined with the result from the larger problem, until the original problem is solved. In Python, a recursive function is one that calls itself."
  assert recursion_explanation("")==""
output: def recursion_explanation(input_string):
      if input_string == "":
          return ""
      return "Recursion is a powerful technique for solving problems. It involves defining a problem in terms of itself. A recursion problem is solved by breaking it down into smaller problems, until the problem is small enough to be solved directly. The result from a smaller problem can then be combined with the result from the larger problem, until the original problem is solved. In Python, a recursive function is one that calls itself."