  We prove an effective version of the Lopez-Escobar theorem for continuous
domains. Let $Mod(\tau)$ be the set of countable structures with universe
$\omega$ in vocabulary $\tau$ topologized by the Scott topology. We show that
an invariant set $X \subseteq Mod(\tau)$ is $\Pi^0_\alpha$ in the effective
Borel hierarchy of this topology if and only if it is definable by a
$\Pi^p_\alpha$ - formula, a positive $\Pi^0_\alpha$ formula in the infinitary
logic $L_{\omega_1,\omega}$. As a corollary of this result we obtain a new
pullback theorem for positive computable embeddings: Let $K$ be positively
computably embeddable in $K'$ by $\Phi$, then for every $\Pi^p_\alpha$ formula
$\xi$ in the vocabulary of $K'$ there is a $\Pi^p_\alpha$ formula $\xi^\star$
in the vocabulary of $K$ such that for all $A \in K$, $A \models \xi^\star$ if
and only if $\Phi(A) \models \xi$. We use this to obtain new results on the
possibility of positive computable embeddings into the class of linear
orderings.
