Parser not reducing expressions

Hi!

I'm writing a custom language plugin for Coq language and I've already implemented a lexer and syntax highlighting
Now I'm trying to write a parser but I can't get it to work, the problem is that it doesn't reduce expressions. It just parses a PsiFile and a list of PsiElement under it, though those elements are well recognized it doesn't group them following the syntax tree.
I simplified the bnf definition to see if I was doing something wrong, but it doesn't work anyways.

This is my simplified bnf:
{
  parserClass="com.qq.coqide.syntax.parser.CoqParser"
  stubParserClass="com.qq.coqide.syntax.parser.GeneratedParserUtilBase"


  extends="com.intellij.extapi.psi.ASTWrapperPsiElement"


  psiClassPrefix="Coq"
  psiImplClassSuffix="Impl"
  psiPackage="com.qq.coqide.syntax.parser.psi"
  psiImplPackage="com.qq.coqide.syntax.parser.psi"


  elementTypeHolderClass="com.qq.coqide.syntax.parser.CoqTokenTypes"
  elementTypeClass="com.qq.coqide.lexer.CoqElementType"
  tokenTypeClass="com.qq.coqide.lexer.CoqTokenType"
}


coqFile ::= sentence *


sentence ::= definition_expr | parameter_expr | comment_expr


definition_expr ::= DEFINITION PARAMETER


parameter_expr ::= PARAMETER IDENT COLON IDENT SEMICOLON


comment_expr ::= COMMENT

This is what gets parsed (using PsiViewer):

Test file contains this text:
Definition Parameter
(* comment *)
Parameter a: a;
Parameter a: a;
Parameter a: a;
Definition Parameter

Any idea what I could be doing wrong? Thanks!

I'm using Idea IC-123.72 and JDK 1.6.0_32 on Ubuntu 12.04.