Wikipedia says the axiom for test in PDL is $$ \langle \psi ? \rangle \phi \leftrightarrow \psi \wedge \phi, $$ but why is this right? (i.e. what does it say?) And what is the corresponding relation $R_{\psi ?}$ on a (regular) frame?
Given an atom $A$, can we say $\langle \psi ? \rangle \phi \in A$ iff $\psi \in A$ and $\phi \in A$?
I know that equivalently one can state the axiom as $[\psi ?]\phi \leftrightarrow (\psi \to \phi)$, implied by $[\psi]\phi \leftrightarrow \neg \langle \psi \rangle \neg\phi$.
NB: the question marks at the and of sentences are real question marks, those inside brackets are test functions.