central_death_rate      Central Death Rate
conditional_death_prob
                        Conditional Probability of Death at Age x
conditional_life_prob   Conditional Probability of Survival at Age x
input_check             Error Handling Function
life_expectancy         Life Expectancy of Age x
lifetable               Lifetable Function
mortality               A sample mortality data
mortality2              A sample mortality data
mortality3              A sample mortality data
number_to_survive       The Number of People to Survive to Age x
person_years            Person Years Lived at Age x
prop_to_survive         Proportion to Survive to Age x
total_years_lived       Total Years Lived From Age x
