Property reactivity does not work with positional/unification constraints

Positional and/or unification constraints are interpreted as either bindings
or constraints, depending on the whether the variable is already bound or not. The user is expected not to have to care about the difference.

...
Person( $name; $age := age )
...

If the pattern type is @propertyReactive, however, the actual decision
between binding and constraint WILL make a difference, possibly forcing
the user to use @watch()