这里讲的是single trailing 和double trailing。
建议看一下 –《SAS Certification Prep Guide: Base Programming for SAS 9》的第20章- 也就是SAS官网上指定的SAS BASE的备考书-论坛里应该也可以找得到。
Chapter 20 Creating Multiple Observations from a Single Record （p.573-606）
•        SAS provides two line-hold specifiers.
o        The trailing at sign (@) holds the input record for the execution of the next INPUT statement.
o        The double trailing at sign (@@) holds the input record for the execution of the next INPUT statement, even across iterations of the DATA step.
•        The double trailing at sign (@@)
o         works like the trailing @ except it also holds the data line in the input buffer across multiple executions of the DATA step
o        typically is used to read multiple SAS observations from a single data line
o        should not be used with the @ pointer control, with column input, nor with the MISSOVER option.
•        It's easy to distinguish between the trailing @@ and the trailing @ by remembering that
o        the double trailing at sign (@@) holds a record across multiple iterations of the DATA step until the end of the record is reached.