Existential Quantifier (exactly one)

The result is true iff the boolean expression is true for exactly one variable binding of the declarations.

Syntax:
exists!
   <var> : <set expression> [, <var> : <set expression> ...]
@
   <boolean expression>
Example:
exists! v: V{structure.Package} @ inDegree{structure.ContainsSubPackage}(v) = 0 and v.qualifiedName=""