public void ValidateDoubleToBeNegative()
        {
            // Given
            var validator = new DoubleValidator(-42);

            // When
            validator.BeNegative();

            // Then
            Assert.True(true);
        }
        public void ValidateDoubleToBeNegativeViolated()
        {
            // Given
            var validator = new DoubleValidator(0);

            // When
            var exception = Assert.Throws <XunitException>(() => validator.BeNegative(because: "that's the bottom line"));

            // Then
            Assert.NotNull(exception);
            var rn = Environment.NewLine;

            Assert.Equal(
                $"{rn}validator{rn}is \"0\"{rn}but was expected to have a negative value{rn}because that's the bottom line",
                exception.UserMessage);
        }