local sin, cos = math.sin, math.cos;
function rotCFrameY(cf, theta)
    local s, c = sin(theta), cos(theta);
    local x, y, z, m00, m01, m02, m10, m11, m12, m20, m21, m22 = cf:components();
    return CFrame.new(x, y, z,
        m00*c+m20*s, m01*c+m21*s, m02*c+m22*s,
        m10, m11, m21,
        -m00*s+m20*c, -m01*s+m21*c, -m02*s+m22*c
    );
end