aspect Nullable {

  // *** API ***

  // A nonterminal has a boolean attribute *nullable*
  // which is true if the nonterminal can derive the empty string.
  inh boolean NDecl.nullable() circular [false];

  
  // *** Implementation ***

  syn boolean Prod.nullable() circular [false];
  
  syn boolean Symbol.nullable() circular [false];
    
  eq Terminal.nullable() {  
     return false;
  }
  
  eq NUse.nullable(){
    return decl().nullable();
  }
  
  eq Rule.getNDecl().nullable() {
    for (int i = 0; i < getNumProd(); i++) {
      if (getProd(i).nullable())
        return true;
    }
    return false;
  }
  
  eq Prod.nullable() { 
    for (int i = 0; i < getNumSymbol(); i++) {
      if (!getSymbol(i).nullable())
        return false;
    }
    return true;
  }
}