input: Write a python function to check if one string is a rotation of another string.
  assert is_rotation('waterbottle', 'erbottlewat') == True
  assert is_rotation('hello', 'llohe') == True
  assert is_rotation('hello', 'olleh') == False
  assert is_rotation('abc', 'cab') == True
  assert is_rotation('abc', 'bca') == True
output: def is_rotation(s1, s2):
      if len(s1) != len(s2):
          return False
      return s2 in s1 + s1