module Option: sig .. end
val none : 'a option React.signal
none is S.const None.
val some : 'a React.signal -> 'a option React.signal
some s is S.map ~eq (fun v -> Some v) None, where eq uses
        s's equality function to test the Some v's equalities.
val value : ?eq:('a -> 'a -> bool) ->
       default:[ `Always of 'a React.signal | `Init of 'a React.signal ] ->
       'a option React.signal -> 'a React.signal
value default s is 
s with only its 
Some v values.
        Whenever 
s is 
None, if 
default is 
`Always dv then
        the current value of 
dv is used instead. If 
default
        is 
`Init dv the current value of 
dv is only used
        if there's no value at creation time, otherwise the last
        
Some v value of 
s is used.
        
- [value ~default s]t= vif [s]t= Some v
- [value ~default:(`Always d) s]t=[d]t
          if [s]t= None
- [value ~default:(`Init d) s]0=[d]0
          if [s]0= None
- [value ~default:(`Init d) s]t=[value ~default:(`Init d) s]t'
          if [s]t= Noneand t' is the greatest t' < t
          with [s]t'<> Noneor 0 if there is no sucht'.