Fixed: [ Issue 35 ]  Imports, options and package can appear in any order.
https://code.google.com/p/protobuf-dt/issues/detail?id=35

com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/Protobuf.xtext

diff --git a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/Protobuf.xtext b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/Protobuf.xtext
index b359efe..0b1c2c8 100644
--- a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/Protobuf.xtext
+++ b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/Protobuf.xtext
@@ -16,9 +16,7 @@
 
 Protobuf:
   (syntax=Syntax)?	
-  (package=Package)?
-  (imports+=Import)*
-  (options+=Option)*
+  ((package=Package)? & (imports+=Import)* & (options+=Option)*)
   (elements+=ProtobufElement)*;
 
 Syntax: