Spahn Pi-closure (Rev #2)

Definition

Let $\dagger$ be a monad on a topos $H$. A morphism $f:X\to Y$ is called $\dagger$-closed if

is a pullback square.

Closure properties

The class of $\dagger$-closed morphisms satisfies the following closure properties:

(1) Every equivalence is $\dagger$-closed.

(2) The composite of two $\dagger$-closed morphisms is $\dagger$-closed.

(3) The left cancellation property is satisfied: If $h=g\circ f$ and $h$ and $g$ are $\dagger$-closed, then so is $f$.

(4) Any retract of a $\dagger$-closed morphism is $\dagger$-closed.