"""
Write a Python program that outputs all possible strings formed by using the characters 'c', 'a', 't', 'd', 'o', and 'g'
exactly once.
"""
# define 'permute' function
def permute(char_pool, current_str):
    # base case: if char_pool is empty, print value of 'current_str'
    if len(char_pool) == 0:
        print(current_str)
        return
    # try each remaining character as the next pick
    for i in range(len(char_pool)):
        # pick character at i: pass down pool without it, and string with it added
        permute(char_pool[:i] + char_pool[i+1:], current_str + char_pool[i])
    

permute('catdog', '')