Given orderings with one rank missing, complete the ordering by assigning the remaining item(s) to the final rank.

## Examples

```
# Orderings of 3 items, when only the best and worst are recorded
orderings <- data.frame(best = c("A", "B", "A"),
worst = c("C", "C", NA))
orderings$middle <- complete(orderings, items = c("A", "B", "C"))
```