Rank syntax support

[dkarvik]

I was unable to generate the syntax for {rank = same; X; Y; Z} without using some dorothy internals.

Here's a small example of what I'd like to do: https://stackoverflow.com/a/44274606/1481316

This is also mentioned in the Dot User's Manual https://www.graphviz.org/pdf/dotguide.pdf on page 16.

Here's how I achieved the syntax, which as far as I can tell is called "Rank":

(defn rank
  [attrs statements]
  {:type ::rank
   :statements (map #'dot/to-ast statements)
   :attrs attrs})

(defmethod dot/dot* ::rank
  [this]
  (let [{:keys [attrs statements]} this]
    (str "{\n"
         (apply
           str
           (for [[k v] attrs]
             (str (#'dot/escape-id k) \= (#'dot/escape-id v) ";\n")))
         (apply str (interleave
                      (map dot/dot* statements)
                      (repeat ";\n")))
         "} ")))

It's a lot like a graph, but without a name, and with the ability to set inline attrs. I decided to front-load the attrs, but ordering might matter, so that might not be ideal.

[handerpeder]

Was wondering how to do this myself. Turns out this is just a subgraph (the keyword subgraph is optional).

The following two statements are equivalent:

subgraph { 
  rank = same; A; B; C; 
} 

{ rank = same; A; B; C; } 

Having sugar for subgraphs would be nice though.