module Option: sig .. end
Events with option occurences.
val some : 'a React.event -> 'a option React.event
some e is map (fun v -> Some v) e.
val value : ?default:'a React.signal -> 'a option React.event -> 'a React.event
value default e either silences 
None occurences if 
default is
        unspecified or replaces them by the value of 
default at the occurence
        time.
        
- [value ~default e]t = vif [e]t= Some (Some v).
- [value ?default:None e]t = Noneif [e]t =None.
- [value ?default:(Some s) e]t = vif [e]t =Noneand [s]t= v.