Formatting for a Ruby-like language #1

Hi,

I was about to implement Wadler's prettier printer, and I'm very happy to find that you already did that. Thanks for this.

I have a few questions:

1. In a language such as Ruby where \n is meaningful, it's important to be able to emit a line break. I don't see how to do that with the current API. Am I missing something?

2. Likewise, in Ruby in many places you can drop a separator when you are on a single line, or just do with a \n otherwise. For instance expressions in a block can be separated by ; , or \n (in which case the ; is useless). Or the then-part of an if can be on the same line with then, or simply separated with \n.
   I can't see a way to express that in the current api. I can see that this violates some invariants stated by Wadler ("As an invariant, we require in (x <|> y) that all layouts in x and y flatten to the same layout"), but I can't see how this invariant would break the correctness of the rendered layout. It does not break the second invariant, that appears to be more critical to get correct results ("we require that no first line of a document in x is shorter than some first line of a document in y; or, equivalently, that every first line in x is at least as long as every first line in y").

3. What are the known uses of this library?

4. I'd like to suggest to modify the documentation (I can provide a PR) which is misleading. It says in several places something like "By default, the class assumes that primitive elements are strings and each byte in the strings have single column in width". This is wrong: since the library uses str.length it does count the number of characters, not of bytes.

Cheers!