5.4.3 Valency Constraints

Every daughter set is a finite set of nodes in the tree:

\forall w\in\VV,\ \forall\rho\in\setof{Roles},\quad\rho(w)\subseteq\VV

The second principle of well-formedness requires that a complement daughter set \rho(w) be non-empty only when \rho appears in w's valency. Additionally, the first principle states that, when it is non-empty, the complement daughter set \rho(w) must be a singleton:

&|\rho(w)|\leq 1\\

In practice, the equivalence above will be enforced using reified constraints which are explained in Section 6.8.

Denys Duchier
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