Which of the following two forms is correct? (And if there's no agreement about that, which one is more common?)
a (optional) parameter
an (optional) parameter
The problem is that the choice between "a" and "an" is based on the spoken form of the phrase ("does the next word start with a vowel sound?"), but there is no spoken equivalent to parentheses.
I.e. I can say either:
an optional parameter
But I can't pronounce parentheses.