This lifetable identity is the same no matter what kind of lifetable is required. You can find it in any demography textbook.
numeric. Vector of age-specific lifetable survivorship.
qx values of age-specific mortality rates. The last value is always 1.0
The vector returned is the same length as
Preston S, Heuveline P, Guillot M (2000). Demography: measuring and modeling population processes. Wiley-Blackwell.