It's always better to understand the rule than to copy an example. The rule to apply here is Section 27g of the Nemeth Code which states that the ELI must not be used with a letter which is not a "single letter." -x is not a single letter by definition (Section 25 defines "single letter") because it is not preceded by a space (Sec.25v). No ELI is used. The rule does not change if a period follows the -x because the x is still not preceded by a space. We often need to challenge our translation software because it cannot read in context. You are smart to question it and to understand its limitations.