First order conjunctive query

Let $\mathcal{P}$ be a program of first order definite rules. A conjunctive query $Q$ to $\mathcal{P}$ is a prenex normal form formula:

$$Q=\exists x_1\dots \exists x_n{A}_1,\dots ,A_m$$

Where $A_1,\dots ,A_m$ is a conjunction of positive atoms. Where $x_1,\dots ,x_n$ are the variables in $A_1,\dots ,A_m$.