Represents a type predicate or assertion signature like `x is Foo` or `asserts x is Foo`. Reflects: `TypePredicateNode` and `TypePredicate`. Example (TS): ```ts function isNum(x: unknown): x is number { return typeof x === 'number' } function assertIsNum(x: unknown): asserts x is number { if (typeof x !== 'number') throw new Error() } ```