type error = | Not_found | The key was not found.  | 
| Not_set | The key was found but had no associated value.  | 
The type of a session error.
These will only be returned by the S.Now.get and S.Future.get operations.
module type Now = sig ... endThe signature for synchronous backends.
module type IO = sig ... endThe signature for a blocking computations.
module type Thread_IO = sig ... endThe signature for blocking computations that can run synchronous computations in a separate thread
module type Future = sig ... endThe signature for asynchronous backends.