anchor.diff:      IS_EXPR;
       GET_EXPR;
       SET_EXPR;
  +    //PRIMARY;
   }
   
   compilationUnit
  
positive.code: primary
      : getterSetterMethodReference
      | prim
      ;	
      
  prim
  
negative.code: // book seems to agree, but the above doesn't work.
  //    : base selector+ -> ^(SELECTOR_LIST base selector+)
  //    | base
      : //base selector* 
      base selector+
      -> ^(EXPR base selector*)
      | base
  