interface readable:
    func read() -> i32

struct fuel_tank:
    liters: i32

func (tank fuel_tank) read() -> i32:
    return tank.liters

func fuel_level(const source: readable) -> i32:
    return source.read()

func entry(liters: i32) -> i32:
    var tank = fuel_tank(liters = liters)
    return fuel_level(tank)