let-Declared Variables

  let s_Initial = 999.0
  ...
  let beta =
    cond:
      gamma              if time <= policy_days0
      gamma + 0.1486983  if time <= policy_days1
      gamma + 0.0717734  otherwise

Variables declared with let are bound to expressions - these may be constants, in the case of s_Initial, or more complex, state-dependent expressions, in the case of beta. As they can depend on state variables, unlike parameters, lets may change their value throughout a model's simulation. Unlike state variables, lets cannot be directly modified in an event's effect section.