Budget-change function
b(E_A,t,E_R,t) = (BI_t,CBI_T)
Maps reference- and adoption-scenario expenditure to periodic and cumulative changes in spending for a specified budget holder.
Periodic budget impact
BI_t = E_A,t - E_R,t
Cumulative budget impact
CBI_T = sum_(t=1)^T BI_t