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