Set Comprehension Expression
A set comprehension expression evaluates to a Set of Tuples. The tuple elements are defined by the expressions in the reportSet part, one tuple for each variable combination of the from part that fulfills the optional boolean predicate in the with part. When no with part is given, all combinations result in a tuple.
The result set does not contain duplicate tuples.
When there is only one expression in the report part, the resulting Set will not contain unary Tuples, but simply the values.