Calculation of life expectancy.

Let
q(x) = probability that a person will die within one year of age x 


l(x) = fraction of those still alive at age x
l(0) = 1			everyone alive at age 0		
l(x) = l(x-1) (1-q(x-1))        number alive at x is number at x-1
				times (1 - probability) of death

d(x) = l(x) q(x)		number of deaths in year x

L(x) = number of lives lived between year x and x+1
L(x) = l(x) - d(x)/2		people who die live on average a half year	

e(x) = life expectancy for a person of age x
e(x) = Sum_{i=x,inf} L(i)  / l(x)


The base q(x) are given in the mortality tables.  They are modified by

q-effective = q (1 + days-lost-per-year/365)

where multiple risk factors are handled by multiplying by multiple factors
(1- days-lost-per-year/365) with the appropriate days lost per year.

This is the "Cox proportional hazards" model.