input: Write a python function to determine the winner in a Rock-Paper-Scissor game using given choices for user and computer.
  assert compare_choices('Rock', 'Scissor') == 'You win!'
  assert compare_choices('Paper', 'Rock') == 'You win!'
  assert compare_choices('Scissor', 'Paper') == 'You win!'
  assert compare_choices('Rock', 'Rock') == 'It is a tie!'
  assert compare_choices('Paper', 'Paper') == 'It is a tie!'
  assert compare_choices('Scissor', 'Scissor') == 'It is a tie!'
  assert compare_choices('Rock', 'Paper') == 'You lose!'
  assert compare_choices('Paper', 'Scissor') == 'You lose!'
  assert compare_choices('Scissor', 'Rock') == 'You lose!'
output: def compare_choices(user_choice, computer_choice):
      if user_choice == computer_choice:
          return 'It is a tie!'
      
      winning_combinations = {
          'Rock': 'Scissor',
          'Paper': 'Rock',
          'Scissor': 'Paper'
      }
      
      if computer_choice == winning_combinations[user_choice]:
          return 'You win!'
      else:
          return 'You lose!'