Who cares about Domain Rules?

namespace BusinessRules.Evolution_Three
{
    /// <summary>
    /// Interface describing the Investor Business Rules.
    /// </summary>
    public interface IIncomeTaxEngine
    {
        /// <summary>
        /// Calculates the tax rate.
        /// </summary>
        /// <param name="income">The investor.</param>
        /// <returns>Returns a double representing the tax liability for the investor.</returns>
        double CalculateTaxRate(double income);

        /// <summary>
        /// Calculates the tax liability.
        /// </summary>
        /// <param name="income">The investor.</param>
        /// <returns>Returns a double representing the tax rate for the investor.</returns>
        double CalculateTaxLiability(double income);
    }
}