A relation is an operand of a relational expression. Its syntax is:

{ [InputList] -> [OutputList] :formula}

* InputList* and * OutputList* are lists of tuple elements.
A tuple element can be:

The * formula* is optional. If it is omitted, no constraints other than
those introduced by the input and output expressions are imposed upon
the relation's variables. Otherwise, the formula describes additional
constraints on variables used in the relation.

In addition to relations, the system can represent * sets*.

When a relation is declared with only one tuple, as in:

then the relation becomes a set. The variables that are used to describe a set ({ [SetList] :formula}

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