Function reduce_by_invariance 

pub fn reduce_by_invariance(
    truth: &[bool],
) -> Option<(InvarianceReduction, Vec<usize>)>

Peel the invariance subspace off a Boolean function: quotient out every direction a with f(x⊕a) = f(x) and return the smaller function on the surviving coordinates, together with the invariance basis that certifies it. None when the invariance subspace is trivial (nothing to reduce).