### 2.6.20. Specialisation

Denotes that constraint ${C}_{\mathrm{𝑎𝑙𝑠𝑜}}$ is a specialisation of constraint ${C}_{\mathrm{𝑒𝑛𝑡𝑟𝑦}}$.

EXAMPLE: As an example, constraint ${C}_{\mathrm{𝑎𝑙𝑠𝑜}}=$ $\mathrm{𝚙𝚊𝚝𝚑}$ is a specialisation of constraint ${C}_{\mathrm{𝑒𝑛𝑡𝑟𝑦}}=$ $\mathrm{𝚝𝚛𝚎𝚎}$. Given a digraph $G$, the $\mathrm{𝚝𝚛𝚎𝚎}$ constraint forces a covering of $G$ by a set of trees in such a way that each vertex of $G$ belongs to one distinct tree. If, in addition, we restrict each vertex to have at most one child we get the $\mathrm{𝚙𝚊𝚝𝚑}$ constraint.