Image via WikipediaSeriously, who are they kidding? We know that extensible markup language (XML), extra large (XL), and extreme backyard wrestling (XBW) all start with an e, not an x. Why make so inexact an exception in an otherwise excellent execution? Is it really so excruciating to admit that one's x-principal opportunities are exceedingly small? (In my Merriam-Webster's Collegiate Dictionary, for example, the X listings don't exceed two pages before being exhausted, and that includes a picture of a xylophone and word history of X ray.)Excluding X from an alphabetical list doesn't expose your inexperience or make you less of an expert. So you don't need to include extraneous ex- examples when none exist. Just expunge that letter from your list entirely.
If you'd like to make an excuse for the exclusion of the xes, feel free to explicitly explain that there are just no x examples to include.
But you really don't need to. Your readers will understand.
