## Each argument Term_1, ..., Term_k is a term.

## A term is either a logical constant (e.g., “Joe”) OR a logical variable (e.g., “?msg”) OR a functional expression of the form:

## LogicalFunction(Term_1, ..., Term_k)

## A functional expression semantically essentially denotes a logical constant.

## A term, atom, or rule is called “ground” when it has no logical variables.

## A fact is a ground rule with empty body.

## A primitive conclusion has the form of a ground atom (compound conclusions are built up from these via logical operators such as AND etc.).

## Semantically, a rule or LP stands for the set of all its ground instances.

## (Observe that a rule body can represent an expression in relational algebra cf. relational DB’s (e.g., SQL).)

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