type Affine = Vec<(f64, f64, f64, f64)>;
A claim or collateral bond at one date: (amount, A, B_x, B_y) per flow, worth Σ amount·A·e^{−B_x·x − B_y·y}.
(amount, A, B_x, B_y)
Σ amount·A·e^{−B_x·x − B_y·y}
struct Affine { /* private fields */ }