<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _check_checksum(msg):
 """Ensure checksum in message is good."""

    checksum = int(msg[-2:], 16)
    for char in msg[:-2]:
        checksum += ord(char)
    if (checksum % 256) != 0:
        raise ValueError("Elk message checksum invalid")