Iterator that yields one array first, then the rest of another

Design an iterator over two integer arrays, a and b, that returns elements in this order:

1. Every element of b, in its original order (duplicates included).
2. Then every element of a whose value does not appear anywhere in b, in a's original order.

Implement a class with this API:

class SubarrayFirstIterator:
    def __init__(self, a: list[int], b: list[int]) -> None: ...
    def has_next(self) -> bool: ...
    def next(self) -> int: ...

• has_next() returns whether another element remains. Calling it repeatedly must not change the result or skip elements.
• next() returns the next element, or raises an error when the iterator is exhausted.
• The iterator should be lazy: do not build the full output list up front.

Constraints: 0 <= len(a), len(b) <= 10^5. Values are arbitrary integers. b may contain values that are not in a.

Example 1

a = [4, 1, 7, 3, 9], b = [7, 4]
output: 7, 4, 1, 3, 9

Example 2

a = [5, 2, 5, 8], b = [5]
output: 5, 2, 8

Both copies of 5 in a are skipped because 5 appears in b.

Reconstructed from the reported question; details may differ from what interviewers ask.