### 2.5.16. Root concept

Constraint ${C}_{\mathrm{𝑒𝑛𝑡𝑟𝑦}}$ is derived from constraint ${C}_{\mathrm{𝑎𝑙𝑠𝑜}}$.

EXAMPLE: As an example, the constraint ${C}_{\mathrm{𝑒𝑛𝑡𝑟𝑦}}=$ $\mathrm{𝚝𝚛𝚎𝚎}_\mathrm{𝚛𝚎𝚜𝚘𝚞𝚛𝚌𝚎}$ is derived from the constraint ${C}_{\mathrm{𝑎𝑙𝑠𝑜}}=$ $\mathrm{𝚝𝚛𝚎𝚎}$. Given a digraph $G$, the $\mathrm{𝚝𝚛𝚎𝚎}$ constraint enforces a partitioning of $G$ by a set of trees in such a way that each vertex of $G$ belongs to one distinct tree. In addition, the $\mathrm{𝚝𝚛𝚎𝚎}_\mathrm{𝚛𝚎𝚜𝚘𝚞𝚛𝚌𝚎}$ constraint distinguishes resource and task vertices, and enforces each tree to contain exactly one resource vertex.