We define an intervals vector to always start with 0, and contain unique ordered time points.

`assert_intervals(x, y)`

- x
what to check.

- y
(

`count`

)

required length of`y`

.

Raises an error if `x`

is not an intervals vector starting with 0.

```
assert_intervals(c(0, 5, 7), 3)
```