The choice of "a" versus "an" depends on the pronunciation of the following word. Mathematical writing may use symbols as nouns or adjectives, and their pronunciation is not always clear. Here is a made-up example.
Consider a (⊗,≤,1) triple such that S forms a commutative monoid with (⊗,1) and x ≤ x ⊗ y for any x and y.
In "a (⊗,≤,1) triple", how would one choose the indefinite article to use before the operator?