检查SELECTION-SCREEN 上面输入的参数是否合适

需求：在选择屏幕上ANLN1，ANLN2,BUKRS 这个三个字段要么同时都有输入，要么都没有输入，如果只输入其中一个，跳出一个message 提醒用户输入其余的参数，请看下面的代码就可以实现该功能了：
TABLES: sscrfields.
AT SELECTION-SCREEN.
  IF sscrfields-ucomm = 'ONLI' OR
  sscrfields-ucomm = 'PRIN' OR
  sscrfields-ucomm = 'SJOB'.
    PERFORM validate_input_para.
  ENDIF.
FORM validate_input_para .
*** Validation Rule 1: The full asset number/sub number/company code must
***                    be provided.  Otherwise the fields should all be
***                    blank.
  IF NOT ( ( p_anln1 IS INITIAL AND
             p_anln2 IS INITIAL AND
             p_bukrs IS INITIAL     )     OR
           ( NOT p_anln1 IS INITIAL AND
             NOT p_anln2 IS INITIAL AND
             NOT p_bukrs IS INITIAL ) ).
*** Issue an error message
*** Additional Code is required to display a message indicating
*** the parameters are not entered properly
    IF p_anln1 IS INITIAL.
      MESSAGE i000 WITH 'Asset Number need to be entered'.
      LEAVE SCREEN.
    ELSEIF p_anln2 IS INITIAL.
      MESSAGE i000 WITH 'Sub Number need to be entered'.
      LEAVE SCREEN.
    ELSEIF p_bukrs IS INITIAL.
      MESSAGE i000 WITH 'Company Code need to be entered'.
      LEAVE SCREEN.
    ENDIF.
  ENDIF.
ENDFORM.                    " VALIDATE_INPUT_PARA