Parallel_arrays.SetTo store a value in a parallel array, we must prove that it does not share unsynchronized state with any other elements. This is the case if its type crosses contention or it lives in a fresh capsule.
set t i a stores the element a at index i. Raises Invalid_arg if i is not in the range [0..length t).
unsafe_set t i a unsafely stores the element a at index i.
insert t i f stores f () at index i. Raises Invalid_arg if i is not in the range [0..length t).