input: Write a function to determine if the first string is a subsequence of the second string. A subsequence is a sequence that can be derived from another sequence by deleting some or no elements without changing the order of the remaining elements. If the first string is a subsequence of the second, return True, else return False.
  assert is_subsequence("abc", "qaxbcy")==True
  assert is_subsequence("abc", "qwerty")==False
  assert is_subsequence("abc", "aabbcc")==True
  assert is_subsequence("abc", "acb")==False
  assert is_subsequence("abc", "")==False
output: def is_subsequence(s, t):
      it = iter(t)
      return all(char in it for char in s)