Precedence constraints (prec) Precedence constraints may appear in a single machine or in a parallel machine environment, requiring that one or more jobs may have to be completed before another job is allowed to start its processing. There are several special forms of precedence constraints:
-if each job has at most one predecessor and at most one successor, the constraints are reffered to as chains;
-if each job has at most one successor, the constraints are reffered to as an intree;
-if each job has at most one predecessor, the constraints are reffered to as an outtree;