Linear temporal logic query

[mdmd]

I am looking to formally describe with linear temporal logic a situation in which γ(X) is true until it is not, i.e. ¬γ(X).
I have the following and would be grateful for your review:
(¬γ(X)) ∪ (γ(X))
definitions:
(B ∪ C)(γ) = (∃i: C (γ_i) ∧ ( ∀j < i:B(γ_j)))
B= ¬ γ (X)
C = γ (X)