The Essence of Datalog (2018)

The predicate symbol for each query is fresh and the terms of the head are just the variables in the body. For example,

gets fresh head atom and can be thought of as

An entire Datalog program is nothing but a collection of rules. Next we evaluate an atom into a list of substitutions by finding facts that fit the template provided by a body atom and capturing the assignments to its variables.

Source: dodisturb.me