input: Write a function to determine the winner of a rock-paper-scissors game.
  assert determine_winner('rock', 'scissors')=='Player 1 won!'
  assert determine_winner('scissors', 'paper')=='Player 1 won!'
  assert determine_winner('paper', 'rock')=='Player 1 won!'
  assert determine_winner('scissors', 'rock')=='Player 2 won!'
  assert determine_winner('paper', 'scissors')=='Player 2 won!'
  assert determine_winner('rock', 'paper')=='Player 2 won!'
  assert determine_winner('rock', 'rock')=='Draw!'
  assert determine_winner('scissors', 'scissors')=='Draw!'
  assert determine_winner('paper', 'paper')=='Draw!'
output: def determine_winner(player1, player2):
      if player1 == player2:
          return 'Draw!'
      elif (player1 == 'rock' and player2 == 'scissors') or \
           (player1 == 'scissors' and player2 == 'paper') or \
           (player1 == 'paper' and player2 == 'rock'):
          return 'Player 1 won!'
      else:
          return 'Player 2 won!'