input: from typing import Union, List
  
  def recursion_problem_explanation(problem_type: str) -> Union[str, List[str]]:
      """
      Explains how to use recursion to solve a given problem in Python.
  
      >>> recursion_problem_explanation('factorial')
      "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."
      """
      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."
output:     if problem_type == 'factorial':
          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."
      elif problem_type == 'fibonacci':
          return "The Fibonacci sequence is a classic example of a problem that can be solved using recursion. The sequence is defined as F(n) = F(n-1) + F(n-2), with F(0) = 0 and F(1) = 1. A recursive function for Fibonacci would call itself to find the values of F(n-1) and F(n-2), then combine them to get the result for F(n)."
      elif problem_type == 'tree_traversal':
          return ["Tree traversal is another common problem solved using recursion.", 
                  "There are three main types of tree traversal: pre-order, in-order, and post-order.", 
                  "In pre-order traversal, the root node is visited first, then the left subtree, and finally the right subtree.", 
                  "In in-order traversal, the left subtree is visited first, then the root node, and finally the right subtree.", 
                  "In post-order traversal, the left subtree is visited first, then the right subtree, and finally the root node."]
      else:
          return "The given problem type is not recognized. Please provide a valid problem type such as 'factorial', 'fibonacci', or 'tree_traversal'."