set or get the level of a page

Each page has a level which measures the distance of the page from the starting page(s). The starting pages have level one. Each page referenced by a page having level N has level N+1.

This method sets or retrieves the level of a page. If the level that the argument specifies is higher than the level that the page already has then the higher value is abandoned, because there is a shorter way getting to this page from the start pages.

If the page has a level and the new level is smaller then the new level is used.

Usage:

              $page->level
or
              $page->level($level)

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