grammatical number - Usage of Parenthetical Plural

I am trying to rewrite two sentences like the following into a single sentence using a parenthetical plural:

1: The rule will add an assertion.
2: The rule will add assertions.

Combining them, the sentence I want would have the form of:

The rule will add an() assertion(s).

I don't know what to put in first parenthesis. Am I allowed to leave it blank? Or should I write 'any' in the first parenthesis? or how else should I write it?