Theorem: Section4sheet1Solutions.subset_def (FormalisingMathematics2024/Solutions/Section04sets/Sheet1.lean)

State before:
X : Type
A B C D : Set X
x : X
⊢ A ⊆ B ↔ ∀ x ∈ A, x ∈ B
Tactic:
rfl
State after:
no goals