Let \( C \) be a circle with centre \( P_0 \) and \( AB \) be a diameter of \( C \). Suppose \( P_1 \) is the mid point of the line segment \( P_0B \), \( P_2 \) is the mid point of the line segment \( P_1B \) and so on. Let \( C_1, C_2, C_3, \ldots \) be circles with diameters \( P_0P_1, P_1P_2, P_2P_3, \ldots \) respectively. Suppose the circles \( C_1, C_2, C_3, \ldots \) are all shaded. The ratio of the area of the unshaded portion of \( C \) to that of the original circle is