1) In "flat_tree_merge_unique" inplace_set_difference is used insted of inplace_set_unique_difference
2) Im "insert_unique", the operation "seq.erase(e, seq.cend());" can invalidate the iterator "it" in the corner case if "it == e", so "it" should not be used in "flat_tree_container_inplace_merge". This invalidation does not show in contiguous memory sequence containers like vector, but it is triggered when using stable_vector and other non-contiguous sequences.
1) In "flat_tree_merge_unique" inplace_set_difference is used insted of inplace_set_unique_difference
2) Im "insert_unique", the operation "seq.erase(e, seq.cend());" can invalidate the iterator "it" in the corner case if "it == e", so "it" should not be used in "flat_tree_container_inplace_merge". This invalidation does not show in contiguous memory sequence containers like vector, but it is triggered when using stable_vector and other non-contiguous sequences.