- Extensionality: as in ZF
- Class comprehension schema: as in Morse-Kelley$\forall x_1,..,x_n\ \exists x \ [x=\{y|set(y) \wedge \phi(y,x_1,..,x_n)\}] $
- The empty class is a set
- Singletons: $\forall x [ set(x) \to set(\{x\})]$
- Boolean Union: $\forall x,y [set(x) \wedge set(y) \to set (x \cup y)]$
Now it is clear that all axioms of Union, Power and Separation over sets are derivable from the above transfer principle, and so $ZC$ is interpretable here. Actaully if we restrict $\phi$ to have no more thatthan three atomic subformulas, we can still interpret the whole of $ZC$. Replacement is not interpretable by this principle. Yet, a minor modification of this principle might succeed in proving replacement over sets, this can be done by changing the closure property to involve only subsets of $HF$, what I call as "proximity closure over HF", so to restate that: