input: Write a function to explain how recursion can be used to solve a given problem in Python. The function should describe recursion as a technique where a problem is defined in terms of itself, and solved by breaking it down into smaller problems until a base case is reached. The results from smaller problems are then combined to solve the larger problem. 
  assert explain_recursion()=="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 explain_recursion()==None
output: def explain_recursion():
      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."