<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_doc_id(document_pb, expected_prefix):
 """Parse a document ID from a document protobuf. Args: document_pb (google.cloud.proto.firestore.v1beta1.\ document_pb2.Document):
 A protobuf for a document that was created in a ``CreateDocument`` RPC. expected_prefix (str):
 The expected collection prefix for the fully-qualified document name. Returns: str: The document ID from the protobuf. Raises: ValueError: If the name does not begin with the prefix. """

    prefix, document_id = document_pb.name.rsplit(DOCUMENT_PATH_DELIMITER, 1)
    if prefix != expected_prefix:
        raise ValueError(
            "Unexpected document name",
            document_pb.name,
            "Expected to begin with",
            expected_prefix,
        )

    return document_id