Added SyntaxNode#write_dot_file

This will output a file in DOT format that you can
feed to Graphviz <http://www.graphviz.org/> to graphically
inspect ASTs generated by Parser#parse. It is a quick hack
and there are many possible improvements. Each SyntaxNode is
assigned a unique integer id in the instance variable @dot_id
using the class variable @@dot_id_counter. The method
SyntaxNode#write_dot does the actual work. Right now it doesn't
ignore any nodes, but one can imagine ignoring nodes only
consisting of whitespace, etc.

lib/treetop/runtime/syntax_node.rb

@@ -1,8 +1,11 @@
 module Treetop
   module Runtime
     class SyntaxNode
+      @@dot_id_counter = 0
+
       attr_reader :input, :interval, :elements
       attr_accessor :parent
+      attr_reader :dot_id
 
       def initialize(input, interval, elements = nil)
         @input = input
@@ -12,6 +15,9 @@ def initialize(input, interval, elements = nil)
             element.parent = self
           end
         end
+
+        @dot_id = @@dot_id_counter
+        @@dot_id_counter += 1
       end
 
       def terminal?
@@ -67,6 +73,26 @@ def inspect(indent="")
             ""
           )
       end
+
+      def write_dot(io)
+        io.puts "node#{dot_id} [label=\"#{text_value}\"];"
+        if nonterminal? then
+          elements.each do
+            |x|
+            io.puts "node#{dot_id} -> node#{x.dot_id};"
+            x.write_dot(io)
+          end
+        end
+      end
+
+      def write_dot_file(fname)
+        File.open(fname + ".dot","w") do
+          |file|
+          file.puts "digraph G {"
+          write_dot(file)
+          file.puts "}"
+        end
+      end
     end
   end
 end