<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def hash_all(strs, digest=None):
 """Returns a hash of the concatenation of all the strings in strs. If a hashlib message digest is not supplied a new sha1 message digest is used. """

  digest = digest or hashlib.sha1()
  for s in strs:
    s = ensure_binary(s)
    digest.update(s)
  return digest.hexdigest() if PY3 else digest.hexdigest().decode('utf-8')