... earlier we said top-level code isn’t allowed in most of your app’s source files. The exception is a special file named “main.swift”, which behaves much like a playground file, but is built with your app’s source code. The “main.swift” file can contain top-level code, and the order-dependent rules apply as well. In effect, the first line of code to run in “main.swift” is implicitly defined as the main entrypoint for the program. This allows the minimal Swift program to be a single line — as long as that line is in “main.swift”.