+val offset_is_free : (owner * Int63.t) list -> bool
+ (** [offset_is_free owners] tests if the offset is free (unused).
+
+ If an offset is free, then it may be discarded without
+ changing the semantics of the disk image. In normal cases
+ this extends to the end of the current block, but blocksize
+ will differ for each owner, so what this really means is
+ tricky in practice. *)
+