`tabular()` fails if not attached

When running the following code...

```r
tables::tabular(extra ~ group * mean, data = sleep)
```

...I get the following error:

```
Error in labelSubset(subset = as.integer(as.factor(group)) == 1L, label = "group") : 
  could not find function "labelSubset"
```

Only attaching the package resolves the issue:

```r
library(tables)
tabular(extra ~ group * mean, data = sleep)
```