empty is the big map with no bindings.

mem ctxt key big_map returns true iff key is bound in the given big_map. Consumes the cost of hashing the given key. Consumes as Storage.Big_map.Contents.mem if the key is not bound yet in the current overlay.

get ctxt key big_map returns the value bound by key in the given big_map. If the key is not bound, None is returned instead. Consumes cost of hashing the given key. Consumes cost as Storage.Big_map.Contents.find in case of the given key is absent in the current overlay. Consumes cost of parsing data if the value is readed from storage.

update ctxt key new_value big_map updates the value bound by key with v if the new_value is Some v. When the new_value is None, delete the entire entry bound by key in the big_map. Consumes cost for hashing the given key. See get_and_update for details.

get_and_update ctxt key new_value big_map works just like update ctxt key new_value big_map except it also returns the old value bound by key. Consumes cost for hashing the given key. This does not modify the underlying storage, only the diff table.