∀x ϕ — for all values of x, ϕ is true
∀x x — contradiction
in propositional logic:
∀x ϕ ≡ ϕ1 ∧ ϕ0
∃x ϕ — there exists a value of x where ϕ is true
∃x x — tautology
in propositional logic:
∃x ϕ ≡ ϕ1 ∨ ϕ0
Build OBDDs using the propositional logic versions. Compute the variables and then take conjunction (∀) or disjunction (∃).