<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def insert_at_frontier(self, operations: ops.OP_TREE, start: int, frontier: Dict[ops.Qid, int] = None ) -> Dict[ops.Qid, int]: """Inserts operations inline at frontier. Args: operations: the operations to insert start: the moment at which to start inserting the operations frontier: frontier[q] is the earliest moment in which an operation acting on qubit q can be placed. """

        if frontier is None:
            frontier = defaultdict(lambda: 0)
        operations = tuple(ops.flatten_op_tree(operations))
        if not operations:
            return frontier
        qubits = set(q for op in operations for q in op.qubits)
        if any(frontier[q] > start for q in qubits):
            raise ValueError('The frontier for qubits on which the operations'
                             'to insert act cannot be after start.')

        next_moments = self.next_moments_operating_on(qubits, start)

        insertion_indices, _ = self._pick_inserted_ops_moment_indices(
            operations, start, frontier)

        self._push_frontier(frontier, next_moments)

        self._insert_operations(operations, insertion_indices)

        return frontier