Design an Expected O(1) Randomized Container

Design a generic container that stores distinct elements and supports:

• insert(element) -> bool: add an element if absent and report whether insertion occurred.
• pop_random() -> element: remove and return one currently stored element, with every stored element having equal probability. Raise a documented empty-container error when no element exists.

Both operations should take expected O(1) time. Explain the representation, update invariants, randomness requirement, and tests. You may assume stored elements are hashable.

Constraints & Assumptions

• Duplicate elements are not stored.
• The random generator can choose a uniformly distributed integer index.
• Preserving insertion order is not required.
• Thread safety is not required initially, but should be discussed as a follow-up.

Clarifying Questions to Ask

• Are duplicates rejected or counted as separate occurrences?
• What exception or return value represents an empty pop?
• Is reproducible seeded randomness needed for tests?
• Must concurrent callers be supported?