Set maths operators

A range of operators to help use sets.

Unlike their same-named Export codes, these operators cannot use Group objects as the target of the operator. Instead they must explicitly target a named set attribute. Thus, count(child) is not valid - it would be necessary first to make a set of the target's children and then count that set. However, count($MySet) would work.

So, if a set attribute doesn't exist, use collect/collect_if to dynamically build a set.

The new operators are:

• count(set)
• max(set)
• min(set)

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