\begin{definition}[Definition:Proper Relational Structure]
Let $A$ be a set or class.
Let $\RR$ be a relation on $A$.
Then $\struct {A, \RR}$ is a '''proper relational structure''' {{iff}}:
:For each $a \in A$, the preimage $\map {\RR^{-1} } a$ of $a$ under $\RR$ is a set (or small class).
\end{definition}