create to_dyn creates a forward declaration. The
to_dyn parameter is used for reporting errors in
val set : 'a t -> 'a -> unit
set t x sets the value that will be returned by
get t to
x. Raises if
set was already called.
val get : 'a t -> 'a
get t returns the
set comp x was called. Raises if
set has not been called yet.