fn defining_equation(
        x: &FieldElement<Self::BaseField>,
        y: &FieldElement<Self::BaseField>,
    ) -> FieldElement<Self::BaseField> {
        (Self::b() * y.square()) - (x.pow(3_u16) + Self::a() * x.square() + x)
    }
Evaluates the equation at (x, y).
Used for checking if the point belongs to the elliptic curve.
Equation: by^2 = x^3 + ax^2 + x.
crates/math/src/elliptic_curve/montgomery/traits.rs