Annot.LazyIAnnotA derivation whose construction is deferred.
Used for the cells of a partitioned Let: building the derivation of the body for every cell up-front is wasteful, since the reconstruction may never explore some of them.
Forcing is shared, but pending substitutions are not: substitute returns a new handle onto the same deferred derivation, with the substitution queued on it. So two handles obtained from a same origin force it at most once, and each then applies its own substitutions to the result.