Clean up rule format in grammar files

lib/propane/grammar.rb

@@ -41,7 +41,7 @@ class Propane
         elsif sliced = input.slice!(/\Adrop\s+(\S+)\n/)
           pattern = $1
           @drop_tokens << Token.new(pattern: pattern, line_number: line_number)
-        elsif sliced = input.slice!(/\A(\S+)\s*:\s*\[(.*?)\] <<\n(.*?)^>>\n/m)
+        elsif sliced = input.slice!(/\A(\S+)\s*->\s*(.*?)(?:;|<<\n(.*?)^>>\n)/m)
           rule_name, components, code = $1, $2, $3
           components = components.strip.split(/\s+/)
           @rules << Rule.new(rule_name, components, code, line_number)

spec/propane_spec.rb

@@ -31,11 +31,10 @@ token int \\d+
 token plus \\+
 token times \\*
 drop \\s+
-Start: [Foo] <<
+Start -> Foo;
+Foo -> int <<
 >>
-Foo: [int] <<
+Foo -> plus <<
->>
-Foo: [plus] <<
 >>
 EOF
     build_parser
@@ -49,18 +48,12 @@ token plus \\+
 token times \\*
 token zero 0
 token one 1
-Start: [E] <<
+Start -> E;
->>
+E -> E times B;
-E: [E times B] <<
+E -> E plus B;
->>
+E -> B;
-E: [E plus B] <<
+B -> zero;
->>
+B -> one;
-E: [B] <<
->>
-B: [zero] <<
->>
-B: [one] <<
->>
 EOF
     build_parser
   end
@@ -69,14 +62,10 @@ EOF
     write_grammar <<EOF
 token a
 token b
-Start: [R1 a] <<
+Start -> R1 a;
->>
+Start -> R2 b;
-Start: [R2 b] <<
+R1 -> a b;
->>
+R2 -> a b;
-R1: [a b] <<
->>
-R2: [a b] <<
->>
 EOF
     build_parser
   end
@@ -85,12 +74,9 @@ EOF
     write_grammar <<EOF
 token a
 token b
-Start: [a R1] <<
+Start -> a R1;
->>
+Start -> b R1;
-Start: [b R1] <<
+R1 -> b;
->>
-R1: [b] <<
->>
 EOF
     build_parser
   end