enum class exponential_decay_spec : unsigned char  {
    center_of_gravity = 1, // Decay = 1 / (1 + value)       -- for value >= 0
    span = 2,              // Decay = 2 / (1 + value)       -- for value >= 1
    halflife = 3,          // Decay = 1 - elog(0.5)/value     -- for value > 0
    fixed = 4,             // Decay = value                 -- for 0 < value <= 1
};