This tests the Apache if-elif-else-endif SSI flow control construct. The core of the code is:

<!--#if expr="$ARACHNA_KEYWORDS = /Flow Control/" -->
<H2>You are looking at the SSI "flow control" page</H2>
<!--#elif expr="$ARACHNA_KEYWORDS = /Date Formatting/" -->
<H2>You are looking at the SSI "date formatting" page</H2>
<!--#else -->
<H2>I don't know what you're looking!</H2>
<!--#endif -->

You are looking at the SSI "flow control" page

You are looking at the SSI "date formatting" page

I don't know what you're looking at!

Besides the match operators, you can test for lexical greater than and less than comparisons, equality, inequality as well as presence and absence of a variable. See the mod_include docs for details on the supported operators.