Allow specifying the start rule name

lib/propane/generator.rb

@@ -66,11 +66,11 @@ class Propane
         tokens_by_name[token.name] = token
       end
       # Check for user start rule.
-      unless @grammar.rules.find {|rule| rule.name == "Start"}
-        raise Error.new("Start rule not found")
+      unless @grammar.rules.find {|rule| rule.name == @grammar.start_rule}
+        raise Error.new("Start rule `#{@grammar.start_rule}` not found")
       end
       # Add "real" start rule.
-      @grammar.rules.unshift(Rule.new("$Start", ["Start", "$EOF"], nil, nil, nil))
+      @grammar.rules.unshift(Rule.new("$Start", [@grammar.start_rule, "$EOF"], nil, nil, nil))
       rule_sets = {}
       rule_set_id = @grammar.tokens.size
       @grammar.rules.each_with_index do |rule, rule_id|
@@ -270,7 +270,7 @@ class Propane
     #   Start rule parser value type name and type string.
     def start_rule_type
       start_rule = @grammar.rules.find do |rule|
-        rule.name == "Start"
+        rule.name == @grammar.start_rule
       end
       [start_rule.ptypename, @grammar.ptypes[start_rule.ptypename]]
     end

lib/propane/grammar.rb

@@ -11,6 +11,7 @@ class Propane
     attr_reader :modulename
     attr_reader :patterns
     attr_reader :rules
+    attr_reader :start_rule
     attr_reader :tokens
     attr_reader :code_blocks
     attr_reader :ptypes
@@ -18,6 +19,7 @@ class Propane
 
     def initialize(input)
       @patterns = []
+      @start_rule = "Start"
       @tokens = []
       @rules = []
       @code_blocks = {}
@@ -63,6 +65,7 @@ class Propane
       elsif parse_module_statement!
       elsif parse_ptype_statement!
       elsif parse_pattern_statement!
+      elsif parse_start_statement!
       elsif parse_token_statement!
       elsif parse_tokenid_statement!
       elsif parse_drop_statement!
@@ -228,6 +231,12 @@ class Propane
       end
     end
 
+    def parse_start_statement!
+      if md = consume!(/start\s+(\w+)\s*;/)
+        @start_rule = md[1]
+      end
+    end
+
     def parse_code_block_statement!
       if md = consume!(/<<([a-z]*)(.*?)>>\n/m)
         name, code = md[1..2]