Conditional Group-based operators

These operators use a scope based on a group. They can work with sets or lists, retaining data type, or may use expressions evaluating as lists or sets.

The group may be any designator from:

• any designator from {children,descendants,sibling,ancestor,all}
• a find()
• a list-based group designator

Such operators are:

• avg_if(group,condition,value)
• collect_if(group,condition,attribute)
• if(condition){action}[else{action}]
• links[(item|group)].kind.[linkType].attribute
• sum_if(group,condition,value)