Unmagic is a library to dynamically type check Obj.t data
to assure safety of their coercions to expected types.
It uses Typerep for the representation of data types.
Secured version of Obj.obj, Unmagic.obj has the following type:

val obj : sharing:bool -> 'a Typerep.t -> Obj.t -> 'a

obj ~sharing (tyrep : ty Typerep.t) o checks the value of Obj.t can be
safely coerced to a value of type ty. If the check succeeds it returns
the coerced value of type ty. Otherwise, it raises an exception Ill_typed.

The parameter ~sharing:true enables sharing and cycle detection:
it avoids repeating type checks of shared nodes and cycles which are
already visited with the same types. It makes the type checking very slow
but is necessary for data with cycles. Type checking may never
terminates for cyclic data with ~sharing:false.