input: Write a python function to compare the user's and computer's decisions in a rock-paper-scissors game.
  assert compare_decisions('rock', 'scissors')=="You win!"
  assert compare_decisions('paper', 'rock')=="You win!"
  assert compare_decisions('scissors', 'paper')=="You win!"
  assert compare_decisions('rock', 'paper')=="The computer wins!"
  assert compare_decisions('paper', 'scissors')=="The computer wins!"
  assert compare_decisions('scissors', 'rock')=="The computer wins!"
  assert compare_decisions('rock', 'rock')=="It's a tie!"
  assert compare_decisions('paper', 'paper')=="It's a tie!"
  assert compare_decisions('scissors', 'scissors')=="It's a tie!"
output: def compare_decisions(user_decision, computer_decision):
      if user_decision == computer_decision:
          return "It's a tie!"
      
      if (user_decision == 'rock' and computer_decision == 'scissors') or \
         (user_decision == 'paper' and computer_decision == 'rock') or \
         (user_decision == 'scissors' and computer_decision == 'paper'):
          return "You win!"
      else:
          return "The computer wins!"