Function eq_one_var 

pub fn eq_one_var<F: FieldOps>(x: F, y: F) -> F

Evaluates the 2-variate multilinear which indicates the equality condition.

eq(X, Y) = X * Y + (1 - X) * (1 - Y)

Over a binary field the cross term vanishes, so this simplifies to

eq(X, Y) = X + Y + 1