`accum` is strict on the update list.

[Superstar64]

Consider the following:

> length $ Vector.accum undefined (Vector.replicate 10 ()) undefined
*** Exception: Prelude.undefined

Since accum never changes the length of the vector, the result here should clearly be 10, however the accumulate list is evaluated.

Here's a workaround for this:

> length $ Vector.generate 10 (Vector.accum undefined (Vector.replicate 10 ()) undefined Vector.!)
10

If it's like this for performance, then this strictness and the workaround should be documented.

[Superstar64]

Actually, on second thought, this might be unavoidable. The result has to be bottom if any element in the update list is outside the index.

> Vector.accum const Vector.empty [(0, 0)]
*** Exception: index out of bounds (0,0)