Recursive grammars

So I was thinking about `combine` a bit recently and it seems to me unlike its haskell older brother it has a problem parsing recursive grammars. Take this example of a parenthesis matching parser that's "translated" from [parsec's documentation](https://hackage.haskell.org/package/parsec):

``` elixir
defmodule Parens do
use Combine

def parser, do: many(parens())
defp parens, do: sequence([char("("), many(parens()), char(")")])
end
```

`Parens.parser` of course results in an infinite loop. I think this works in parsec, because everything is a thunk in haskell so the particular parsers are only evaluated as needed. In this case however we're trying to build a whole infinite parser at once. Or maybe there's a way to achieve this currently?