Recall a continuous map is *separated* if its diagonal is closed. This is equivalent to the fibers being relatively Hausdorff in the total space.

**Proposition.** Suppose $\mathrm P$ is a class of continuous maps satisfying the following conditions.

 - Closed under composition.
 - Stable under pullback.
 - Contains closed topological embeddings.

Then $g\circ f$ lies in $\mathrm P$ and $g$ is separated, then $f$ has $\mathrm P$.

*Proof.* Form the obvious pullback diagrams and play around.

**Question.** What is the topological intuition behind this proposition? How does the separation property of fibers somehow allow to "cancel" separated maps?