Test multiple starting rules

lib/propane/grammar.rb

@@ -242,8 +242,11 @@ class Propane
     end
 
     def parse_start_statement!
-      if md = consume!(/start\s+(\w+)\s*;/)
-        @start_rules << md[1] unless @start_rules.include?(md[1])
+      if md = consume!(/start\s+([\w\s]*);/)
+        start_rules = md[1].split(/\s+/).map(&:strip)
+        start_rules.each do |start_rule|
+          @start_rules << start_rule unless @start_rules.include?(start_rule)
+        end
       end
     end