Let-Expressions

You can bind an expression to an identifier locally by using a let form:

let <id> = <expression> |
  <formula>

This is useful to avoid code bloat due to re-use. E.g., if s is a state:

let s2 = Traces.nextState[s] |
  canTransition[s, s2]

Using let in the evaluator

A let expression can be useful when debugging a model using Sterling's evaluator. E.g., if you want to evaluate an internal subformula for a specific value of a quantifier:

some p: Person | some p.spouse

you can check individual values by directly substituting (e.g., some Person0.spouse) but this is tiresome if the variable is used in multiple places. Instead, consider using let: 

let p = Person0 | some p.spouse

Concrete atoms

This trick (referring to concrete objects) is only usable in the evaluator, because at that point a specific instance has been identified.