Operations on lazy tree contents.
hash t is the hash of the contents value returned when t is forced successfully.
caching
cache regulates the caching behaviour regarding the node's internal data which are be lazily loaded from the backend.
cache defaults to true which may greatly reduce the IOs and the runtime but may also grealy increase the memory consumption.
cache = false doesn't replace a call to clear, it only prevents the storing of new data, it doesn't discard the existing one.
force t forces evaluation of the lazy content value t, or returns an error if no such value exists in the underlying repository.
Equivalent to force, but raises an exception if the lazy content value is not present in the underlying repository.
val clear : t -> unitclear t clears t's cache.