Generates a lookup vector such that `x_at_y(x, y)[y] == x`

. Particularly useful
for generating lookup tables for nested indices in conjunction with `compose_data()`

.

`x_at_y(x, y, missing = NA)`

- x
Values in the resulting lookup vector. There should be only one unique value of

`x`

for every corresponding value of`y`

.- y
Keys in the resulting lookup vector. Should be factors or integers.

- missing
Missing levels from

`y`

will be filled in with this value in the resulting lookup vector. Default`NA`

.

`x_at_y(x, y)`

returns a vector `k`

such that `k[y] == x`

. It also
fills in missing values in `y`

: if `y`

is an integer, `k`

will contain
entries for all values from `1`

to `max(y)`

; if `y`

is a factor,
`k`

will contain entries for all values from `1`

to `nlevels(y)`

.
Missing values are replaced with `missing`

(default `NA`

).

```
library(magrittr)
df = data.frame(
plot = factor(paste0("p", rep(1:8, times = 2))),
site = factor(paste0("s", rep(1:4, each = 2, times = 2)))
)
# turns site into a nested index: site[p] gives the site for plot p
df %>%
compose_data(site = x_at_y(site, plot))
#> $plot
#> [1] 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
#>
#> $n_plot
#> [1] 8
#>
#> $site
#> [1] 1 1 2 2 3 3 4 4
#>
#> $n_site
#> [1] 4
#>
#> $n
#> [1] 16
#>
```