The classical boolean operators are: not(prefix negate), and(conjunction), or(disjunction) and => (Implies). The boolean values are: true and false. Notice that the constraints must be boolean expressions.
let x: Bool = true
let y: Bool = false
let z: Bool
constraint c = (x or not z and true => true)
If the verbose option is set to 2 (at least), we can see that the boolean expression priority behave as expected:
smt-lang -f boolean.sl -v2
...
constraint c = ((x or ((not z) and true)) => true)
...