Some concepts in TypeScript can not be described by types, but can be expressed through type constraints. For example, you can't define a type for positive numbers, but you can check whether a number literal type is positive. One of the patterns for applying such constraints is constrained identity function (CIF). A CIF takes one parameter, infers its type, performs additional checks and returns the parameter unmodified. ```ts // Ensures `n` is a positive number function positiveby Evgeniy @bgenia