How to set up a check for variables using Apache Server-Side Includes?

This is what I'm trying to do in my .shtml page:

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if there is a variable for PREV (if it exists)

display <a href="PREVvalue">Previous</a>

if there is a value for NEXT

display <a href="NEXTvalue">Next</a>

elseif

don't display anything

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I know how to set the variable and pass it to the .shtml page, but I can't figure out how to check if the PREV or NEXT variable has a value, and only show the link if there's a value.