Returns the cause-specific single-decrement survival fraction. The single-decrement survival fraction \(\sdsfrac_\cause(x)\) at age \(x\) is related to the cause-specific force of mortality \(\force_\cause(x)\) through the expression \begin{equation} \sdsfrac_\cause(x) = \exp\left(-\int_0^x \force_\cause(a)da\right) \end{equation} Cause is a comma-separated list of cause codes, possibly prefixed by '~' to denote the complement.