Any ideas?
Show that the set of all x such that x is in A and x is not in B exists.
Hi,
what are $\displaystyle A,B$? sets?
if they are sets, then the axiom schema of restricted comprehension, namely the axiom $\displaystyle \forall b\forall a \exists z \forall x (x\in z \leftrightarrow (x \in a\,\&\, \phi(x,b)))$ where $\displaystyle \phi(x,b)$ is the formula $\displaystyle x \not \in b$, ensures existence of the set you described (just evaluate $\displaystyle a$ to be $\displaystyle A$ and $\displaystyle b$ to be $\displaystyle B$)