我刚开始学习sas，我的数据集中有23个变量，我的程序如下：data merger.sample3;
set merger.sample;
keep tran_id fyear first_annoucement_date buyer_stock buyer_name buyer_industry seller_stock seller_name seller_industry
if_success unit_price tran_capacity tran_value tran_size if_control if_same_goverment if_related interlock sum_interlock
type business_type restur_type;
run;
log显示：
WARNING: The variable restur_type in the DROP, KEEP, or RENAME list has never been referenced.

这是怎么回事呢？sample数据集中有restur_type这个变量呀。
求大神解答。