Booleans represents truth values of for instance guards and other conditions and properties. The only two possible values are true (condition or property holds) and false (condition or property does not hold). The default value of booleans (bool type) is false. Several standard logical operators are available to work with booleans, including the following:

not x       // inverse

x and y     // conjunction (both x and y must hold)
x or y      // disjunction (either x, y, or both must hold)
x => y      // implication (if x holds, y must hold)

x = y       // equal to
x != y      // not equal to

The condition x < 3 evaluates to true if x is less than 3 and to false if x is 3 or larger than 3. The result is thus a boolean value.

Conditions can be combined. x >= 3 and x <= 9 means that the value of x must be both at least 3 and at most 9. x >= 3 or x <= 9 means that the value of x must be at least 3, at most 9, or both. Since the condition is always satisfied (it always evaluates to true), condition true can be used instead of x >= 3 or x <= 9.