How to search a customed theorem with lean-gym?

[liebenxj]

Hi, I wonder how to search a customed theorem with lean-gym? For example, ∀ (a b : ℝ), ∃ x, (a + b) ^ 3 = a ^ 3 + x * a ^ 2 * b + 3 * a * b ^ 2 + b ^ 3. Thanks!