The Clojure map representation of a Datom.

Each entity in the DB is uniquely specified its Entity ID (EID).  Indeed, allocation of a unique
EID is what 'creates' an entity in the DB.

An EntitySpec is used to uniquely specify an entity in the DB. It consists of
either an EID or a LookupRef.

If an entity has an attribute with either :db.unique/value or :db.unique/identity, that entity
can be uniquely specified using a lookup-ref (LookupRef). A lookup-ref is an attribute-value pair
expressed as a tuple:  [ <attribute> <value> ]

A map returned by a successful transaction. Contains the keys 
:db-before, :db-after, :tx-data, and :tempids