I know I must be getting stuck on notation. However, I'm having trouble following the logic in Example 1.2 in https://arxiv.org/pdf/cs/0611018.pdf. They define what preserving a propositional formula means.
So, I'm trying to understand a basic example. Take X = A or (Not B) or (Not C) as an example Horn Clause. Define the assignments f1 and f2 by f1(A) = f1(B) = f1(C) = true and f2(A) = f2(B) = f2(C) = false. Both f1 and f2 satisfy X.
What does it mean to say AND preserves X with respect f1 and f2?