If you have a set of constraints that you use often, or that you'd like to give a name to, you can define a predicate using the pred keyword. A predicate has the following form:

pred <pred-name> {

Newlines between formulas in a pred will be combined implicitly with ands, helping keep your predicates uncluttered. Predicates can also be defined with arguments, which will be evaluated via substitution. For instance, in a family-tree model, you could create:

pred parentOrChildOf[p1, p2: Person] {
  p2 = p1.parent1 or
  p2 = p1.parent2 or
  p1 = p2.parent1 or
  p1 = p2.parent1

and then write something like some p : Person | parentOrChildOf[Tim, p]. Predicates may be used like this anywhere a formula can appear.