This function returns the depth of a context tree, i.e. the length of the
longest context represented in the tree.

## Arguments

- ct
a context tree.

## Value

the depth of the tree.

## Examples

```
dts <- c(0, 1, 1, 1, 0, 0, 1, 0, 1, 0)
dts_ctree <- ctx_tree(dts, min_size = 1, max_depth = 3)
## should be 3
depth(dts_ctree)
#> [1] 3
```