Stdune.FdeclSourceForward declarations
create to_dyn creates a forward declaration. The to_dyn parameter is used for reporting errors in set, set_idempotent and get.
set t x sets the value that will be returned by get t to x. Raises if set was already called.
set_idempotent ~equal t x sets the value that will be returned by get t to x. Raises if set or set_idempotent was already called with a value that is not equal to x.