Subset calls not properly recursively substituted

[brodieG]

> val.toks <- list(INT)
> vet(val.toks[[1]], 1L)
[1] "`1L` should be type \"language\" (is \"integer\")"

[brodieG]

There doesn't seem to be a great solution here since the problem is that val.toks is a list, so it will not get recursively substituted. One potential question to explore is whether we want to allow recursive substitution with expressions so that we could have a "list" of vetting expressions.