For each rule justifying a local mother-daughters configuration, all occurrences of the ↑ symbol (called a metavariable) in the functional annotations of the daughters are replaced by the mother node, and for each of the daughter categories, all occurrences of the ↓ metavariable in its annotations are replaced by the corresponding daughter node.
3 the ↓ of the annotations on a daughter category of a rule and the ↑ of the annotations of the rule that further expands that category are always instantiated with the same node.