类型1：该类文本的页行数固定。
Type 1: Pages of this type of text have a fixed number of lines.
与每页行数固定的情况一样，仅当页在指定范围之内才写输出。
As in the fixed-lines-per-page case, write output only if the page is within the specified range.
selpg通过以下方法记住当前页号：如果输入是每页行数固定的，则selpg统计新行数，直到达到页长度后增加页计数器。
Selpg keeps track of the current page number as follows: If input is fixed-lines-per-page, it counts newlines until the page length is reached and increments a page counter.
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