Axiom of power set
Formal statement

The subset relation ⊆ {\displaystyle \subseteq } is not a primitive notion in formal set theory and is not used in the formal language of the Zermelo–Fraenkel axioms. Rather, the subset relation ⊆ {\displaystyle \subseteq } is defined in terms of set membership, ∈ {\displaystyle \in }. Given this, in the formal language of the Zermelo–Fraenkel axioms, the axiom of power set reads:

where y is the power set of x, z is any element of y, w is any member of z.

In English, this says:
oulipo:avoid_letter[o]